If you’re running into trouble here are some simple steps to try.
Make sure Printopia is turned ON and says "Shared".
Make sure your iPad/iPhone and Mac are both on the same Wi-Fi network.
Make sure you're running the last version of Printopia 2. Printopia 2.1.23 is the final version of Printopia 2 to be released. To check your Printopia version, look for the version number in the preference pane below the Printopia logo.
Consider upgrading to Printopia 3. Printopia 3 was released in October 2017 and contains hundreds of improvements along with support for the latest versions of macOS and iOS. Upgrade pricing is available for users who purchased Printopia 2 in the past.
Update your Mac to OS X Yosemite version 10.10.4 or later. OS X 10.10.4 includes several networking reliability improvements and is recommended for all OS X Yosemite users. If you're running any version of macOS between 10.10 and 10.10.3, upgrading is very likely to resolve your problem. You do not need to upgrade immediately if you are running a version of OS X prior to version 10.10.
Reset your Apple Airport, Time Capsule, or other wireless access point/router. This is especially important if you've upgraded OS X, as it can remove cached, invalid records preventing sharing from working correctly. You may reset these devices using the Airport Utility (for Apple devices) or simply unplugging them for a few seconds.
Check your Wi-Fi configuration. If you're in an apartment complex, office building, or you're at a trade show or other crowded venue, your Wi-Fi network may be experiencing heavy interference. For best performance in this environment we recommend configuring your network to use the 5GHz band exclusively, using HT20 or "narrow" channels. Avoid using 802.11ac or the "wide" HT40 mode as the wider channels are faster but much more susceptible to interference. Finally, let the Wi-Fi access point choose the 5GHz channel automatically. This will allow it to choose a DFS channel if supported by your hardware and permitted at your location. DFS channels are often less crowded than the rest of the 5GHz band.
Disable sleep. Your Mac must be awake for Printopia to work properly. In Printopia version 2.1.12 or later, under Advanced Settings there is a new option to "Keep machine awake while connected to line power". Checking this box will still allow your display to sleep, but will prevent your Mac from going to sleep entirely, causing your shared printers to disappear from the network. You do not need this feature if you are able to get "Wake on Demand" working on your network.
Make sure your printer works normally by printing from your Mac. If not, open System Preferences, click on "Printers and Scanners", select your printer, and click "Open Print Queue...". See if your printer is paused, or there's a job stuck in the queue. Click the "Resume" button if your queue is paused. If your Mac still cannot print to the printer in question, try removing your printer and re-adding it. This reloads cached driver and network information for your printer and can resolve many printing issues.
Check your firewall settings. (System Preferences-> Security-> Firewall). If your Firewall is turned on, click "Advanced", and ensure that "Printopia Server" is listed as allowing incoming connections. Also make sure that the "Block all incoming connections" checkbox is not checked. Alternatively, you may wish to temporarily turn off your firewall for troubleshooting purposes. If you are using Intego's firewall software, ensure that "Client, local server" is selected rather than "Client only" mode.
Power cycle your iPhone or iPad. Unfortunately, with iOS 8, this resolves problems like this more often than not. On an iPhone 6 or 6 Plus, hold the side button down for a few seconds until the "Slide to Power Off" screen appears. On an earlier iPhone, hold the top button instead.
Disable printer sharing. Make sure your Mac's built-in printer sharing is turned OFF. Open System Preferences, click Sharing, then make sure the Printer Sharing option is not enabled.
Make sure your Mac is connected to the same network, either wirelessly or directly with an ethernet cable. Your iOS device must be able to communicate with Printopia in order for it to work. Also, make sure you're on your main network, and not your "guest" network if you have one configured. The "guest" network feature on many wireless access points prevents services like AirPrint from working properly.
Comcast customers (United States): Make sure you're not using the "xfinitiwifi" Wi-Fi network. This is a guest mode network and will not allow you to access your printers. In fact, if you're using a Comcast-provided wireless router and you have the "home hotspot" feature on your router we strongly recommend disabling it to increase performance.
If you use your own Wi-Fi router in addition to one supplied by your ISP, make sure your own Wi-Fi router is configured in "bridged" mode. Otherwise, you may end up with what's called a "Double-NAT" configuration, where internet connectivity will still work for all devices, but some devices within your home may not be able to communicate with each other directly. To check an Apple Airport configuration, open the Airport Utility, select your Airport, click "Edit" to edit the settings, and in the "Network" tab you should see "Router Mode: Off (Bridge Mode)". If you do not see this, and see "DHCP and NAT" instead, change your router mode to "Off (Bridge Mode)".
If you have a dual-band wireless access point, some access points do not allow devices on the 5 GHz band to communicate with devices on the 2.4 GHz band, due to either a misconfiguration or a software flaw. You could try making sure you're connecting to the same band if your networks have different SSIDs, or temporarily turning one band off to force all clients to connect to the other. If this resolves the issue, contact the manufacturer to see if there's a firmware update available to fix this or upgrade your router.
If you're still not seeing any printers, try opening your Wifi network settings on your iPad/iPhone. Make sure you remember your Wifi password, then tap "Forget This Network". Now, attempt to re-join your Wifi network, and re-enter your Wifi password. This clears out some cached information that can prevent printing services from working properly.
Check your iPhone or iPad's Print Center. To do this, double-press the device's home button to reveal the task tray. If you see a "Print Center" icon, usually to the left of your currently open apps, this means there is an item in the print queue that has not yet printed. Tap print jobs to view their status, and cancel any jobs that may be stuck in the queue. (If you don't see a Print Center icon, it just means that there's nothing in the queue.)
Check your printer's print queue on your Mac for status updates. To see all print jobs, choose "Show Completed Jobs" and "Show Everyone's Jobs" from the "Jobs" menu.
Try rebooting the iPhone or iPad. This may clear out cached information about printers which may have become invalid.
Try the "Send To Mac" printer option. If this works, but your real printer does not work, this means it's likely a printer-specific issue and not an issue with your network.
Be sure to clear the iPhone or iPad print queue before testing again.