It is rather difficult to tell without actually seeing the structure of the tables.
One quite common reason for relational integrity problems is different data types - accidentally using a text field in one table and then using an integer field for the same data in the other table. So that would be my first port of call.
One of the very best sites for Access problems is at
and I can recommend it highly. It has helped me many times - and they're usually fairly quick there.
Make sure that you give good details - and don't forget to mention which Access version you're on.