In the design of the form go to the record source property and open up the underlying query in datasheet view. Next try and edit a record. If it does not allow you to do so, which I suspect, you have a non updatable query. This will normally be due to the links and indexes you have on the tables. If I does allow you to edit the query then the issue lies elsewhere. Try and answer the my first question before proceeding further.
CodeMaster: