If you’re running into trouble here are some simple steps to try.
Printopia is now an app. Printopia still runs in the background even after you've quit the app, and can be found in your Applications folder now instead of System Preferences.
Licensing has changed. Printopia 3 license keys do not work with Printopia 2 and vice versa. To use Printopia 3, you'll need to upgrade your existing Printopia 2 license key. To determine if your key is eligible for a discounted upgrade click here.
Printopia 3 requires Mac OSX 10.7 or later. If you're using an earlier version of macOS you'll want to continue using Printopia 2.
Installing Printopia 3 automatically removes Printopia 2 and Printopia Pro if present. Your settings will be preserved in case you'd like to revert back, but you don't need to manually remove the older versions of Printopia from your machine.
Open Printopia, click the "Overview" tab, and make sure it is turned on.
Make sure Printopia is up to date. Printopia 3.0.23 is the latest version. The recommended way to update is to open the Printopia application, then choose Check for Updates... from the Printopia menu at the top left-hand corner of your screen. This is the most secure and convenient way to update your software.
Make sure your iPad/iPhone and Mac are both on the same Wi-Fi network. If your Mac is wired, be sure it is on the same network segment as your Wi-Fi network.
If your printer is showing as offline, or is not printing, check the print queue. Go to System Preferences → Printers & Scanners, select your printer, and click Open Print Queue.... Check to 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 you 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.
Make sure your system is up to date. We generally recommend running the latest version of macOS your machine supports.
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.
Disable sleep. Your Mac must be awake for Printopia to work properly. Go to Server → Advanced Settings... → General, and make sure the "Keep machine awake while connected to line power" checkbox is enabled. Enabling this will still allow your display to sleep but will prevent your Mac from going to sleep entirely.
Check your firewall settings. Go to System Preferences → Security & Privacy → 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. You may wish to temporarily turn off your firewall for troubleshooting purposes. If you are using Intego or other third-party software to configure your Mac's built-in firewall, ensure that "Client, local server" is selected rather than "Client only" mode.
Disable printer sharing. Make sure your Mac's built-in printer sharing is turned OFF. System Preferences → Sharing and 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.