In my experiments I add a message like “Please wait until you see the green Thank you for your patience message”.
One way of confirming that this is the issue, is to use my VESPR Study Portal.
In the portal you can see the number of confirmed and lapsed participants separately. To the best of my knowledge, you can’t reach the debrief page (and have your participation confirmed using the correct combination of participant and session values) until after the data has been saved.