Is there any error that shows on the page? Have you made any changes to the default “Thank You” page that our plugin creates? This page’s URL is set in the following field of the settings menu:
Checkout Result Page URL
Has that been altered in any way?
Enable the debug logging option then do a test transaction so you can see if the full process is going through or something is failing somewhere:
Using the Debug Log to Solve Transaction Related Issues on a Site