Is the application using an auto-enter serial field or is it using some other method to generate the sequence number?
If a user adds a record (auto-enter) then deletes the record there would be a skip in the numbering. Filemaker does not add the number back when the record is deleted.
It is a Number field, set to Auto Enter, Serial. I don't have admin rights so I had to open an earlier version of the database.
The new record is not being deleted. It just occassionaly skips a number and goes to the next.
I suspect that this is not the case, but that a record is being deleted somehow or "lost". Auto-entered serial numbers have been very reliable over many different versions of the software.
You might also try rebuilding the table's indexes as a corrupted index could fail to correctly find or sort records and this can create the apparance of a gap.
For your version, (There are simpler ways for current versions), the simplest way to rebuild all field indexes is to save a clone of your file and then import all data from your current copy into the clone. All the indexes in the table will be rebuilt from scratch during the import.
We'll have a look at the indexes.