There can be several reasons for this.
- GA needs one or two days to show events
- Users use browser add-ons to prevent tracking
- GA is not set up correctly
- Your thank you page is a redirect to some page outside Campaignion
- Some bug is happening
How does Campaignion GA track events?
It's not trivial to find the right moment to trigger an GA event and we put a lot of thought into the question. If the event is triggered when clicking the submit button, it's too early: maybe the form has not been filled in correctly and then it won't be submitted at all. So it's not a very good idea to trigger the GA event before you're absolutely sure the form really has been submitted correctly. And the easiest way to make sure is realizing you made it to the thank you page.
Only that doesn't work in a case where the thank you page is a redirect, because it knows nothing of the form.