Thanks for coming back to the Online Community, mark-adey.
I’m happy to know you’re able to find the missing transaction. The steps you shared with us will also help other customers who are experiencing the same issue.
For future reference, check this article about the Flat Rate Scheme.
If you have any questions or other concerns, please know the Community space is the best place for help in QuickBooks. Wishing you the best with your business endeavors.