Matt,

If your form is based on a query that uses multiple tables, it is normal
that in some cases you can't add records. This happens for example, when the
query does not initialize the foreign key value of a related table. If you
open the query in datasheet view and try to enter a new record there, you'll
see why.

Try this:
Make a copy of the query and save it. Run it from the database window. Go to
the last line and enter the fields that exist only in your form. Chances are
there is a field that is left empty that shouldn't be empty. Access will
refuse to add the record in that particular case.

If this is the case then you'll need to design your form in such a way where
the primary table gets populated first then the secondary table after.