Which is the Bound Column of the combo?
Is this the display column?

When the combo's NotInList event fires, the NewData argument passes the
value the user typed into the combo. If this is not the value that gets
stored in the Bound Column, your approach won't work.

For example, a typical combo has properties like this:
   Control Source        CategoryID
   RowSource             SELECT CategoryID, CategoryName FROM tblCategory;
   Bound Column        1
   Column Widths       0
Then the user enters a new category (text), which is *not* what goes in the
CategoryID column (number.) If this is what you are doing, the NotInList
event will not work. You need to provide another mechanism for adding new
items, such as a command button beside the combo, or its DblClick event.

Access 2007 allows you to specify the form to use to add items to the
combo's source table. You can then right-click the combo to add items.