The OP said that the Form is based on a Table, not a multi-table Query, although I'm not really sure what "The data that appears in the data sheet view of the form is populated based on the combo box data..." means.
If you got to Properties - Format and scroll down near the bottom, there's a Property named 'Split Form Datasheet;' is it set to Read-Only? If so, change it to Allow Edits and see if that helps.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007