I am guessing that your Form is based on a query?
Is it important to note that not all queries are updateable. See: Microsoft Access tips: Why is my query read-only?
A good test is to go to the underlying query, and see if you can update records directly in that query.
So, are you saying that you have a Form/Subform relationship going on here?
How are they related?
What exactly are you trying to update?