The most common reason is that the form's Recordsource is a query which isn't
updateable. Open the form in design view; view its Recordsource property;
click the ... icon by it to open a query design window. If you open the query
as a datasheet, does it have a *> blank new record at the bottom? If not, it's
not editable.
You could open it in SQL view and post the SQL text here if you'ld like help
finding out why it's not.