So, the form is bound to the table by a query or just by the table directly? It should be via a query (you shouldn't go with tables directly, but if you are it wouldn't cause your problem). Your problem is likely caused in the fact that you have your form bound to a query which is not updateable and a query is made non-updateable by several possible factors:
1. There is a calculation in the query itself
2. There are concatenated fields
3. There is a custom column which doesn't appear in the table(s)
4. There is no primary key defined in one, or more, tables that are joined in the query.
If that doesn't help diagnose it, I would say we need to either get you to post your database or let me get it via email so I can see the exact structure of what you are trying to describe.