There could be many reasons why this is occurring including slow internet connections 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, you should:
Clean up the cache and cookies in your currently used browser.
Try using a different Internet browser - preferably Google Chrome.
Try logging in from a different device
Here's how to clear your cache in Google Chrome.