If you enter a new record or change an existing one, then you exit the form
by clicking the close (x) button at the top right corner, and a problem
occurs that prevents the new or changed reciord being saved (eg. a missing
required item), Access will discard the changes >invisibly<, without giving
any message. So, try doing it again, but this time save the new or changed
record by pressing shift-enter. Perhaps it will then complain with a message
saying why the changes can't be saved.
Also, as the other respondent said, not all queries are updatable. Call up a
record on the form, & try to edit a field. Checdk down to the status bar at
the bottom left of the screen. If you see the message "Not updatable" (or
similar), there is your answer!
HTH,
TC