Many things "could" be wrong. Some of them are:
-You could not have full rights to the location that the database resides
-The database could be marked read-only or the location is resides could be
out of disk space.
-There could be Access Security applied to it preventing you from adding to
it
-You could be trying to add a record that violates referential integrity
-You could be trying to add to it through a query that is non-updateable
Are you getting any error messages or a message in the status bar?