Dynaset inconsistant updates will not usually allow modifications or adding
new records. Look back at the query or table you are using for the source of
the form. Is it editable, or have the arrow and asterisk at the bottom. If
not, it is not editable and you need to fix that first. Make sure all your
keys are set correctly, all your tables and queries are updateable, then set
your form to dynaset only.

It is most likely the query produces an uneditable recordset.  This will for
certain happen if the query is a Total query or a Union query.  It can also
happen when multiple records are created from one table are created because
of a many to one join.  Most likely, you will need to restructure you query
to create an editable recordset.

You can tell if the recordset is editiable by opening the query in datasheet
view.  If the add record nav button (with the * on it) is greyed out, it
can't be edited.