If you miss the 21-day registration deadline for short-term lets, you might be subject to enforcement actions regarding your property. Unfortunately, the available council information does not specify the exact consequences of missing this deadline.
For guidance on how to proceed, I recommend contacting West Lothian Council's Licensing Team directly. They can provide you with specific information regarding your situation. You can reach them at licensingservices@westlothian.gov.uk or by calling 01506 281632.