There are a few different reasons why gaps may appear in the Receipt numbering through a Friends Asking Friends report:

1. If a receipt was Voided due to a Reissue, a new receipt number would have been assigned and the old receipt number would no longer display
2. If a transaction was reversed
3. The receipt number in question may have been taken through another FAF event

If you are still unable to locate the missing receipt numbers:
Chat with Support and reference this article.