Gaps in the Transferor's Transaction Serial Number ("TTSN") sequence are not violations in and of themselves, however they may raise questions upon inspection that may lead to violations.
Gaps in your TTSN's can only happen when a TTSN is generated and not used. Every time FastBound generates a TTSN it will be unique and it will be +1 from the previously generated TTSN. FastBound cannot skip numbers.
If you have gaps in your TTSNs and you want to fix them, you can manually type in the missing TTSNs, instead of generating a new one, to use them.
Keep in mind that if you remove a TTSN and type in a different number, the number you removed will never be issued again.