There could be many reasons why this is occurring, including slow internet connection or server maintenance. However, the most common reasons are using an incompatible browser or caching issues.
Printify currently works best in Google Chrome and our Mockup Generator is not designed for mobile functionality.
In order to troubleshoot this issue, please try the following:
Clean up the cache and cookies in your currently used browser (here's how to do it for Google Chrome)
Try using a different Internet browser - preferably Google Chrome.
Try logging in from a different device.