There are so many factors so it could be:
1. Referential Integrity - the ID in the related records must match
2. You may have set Validation Rules that disallows certain values being entered
3. The ID may need to be AutoNumber or Number depending on the situation
4. You may have set the Required property of certain fields and it throws and error if you try to insert Null
5. The datatypes of one or more fields don't match
6. Input Mask violations
There must be other instances but those are the few I can think of at the moment.