From the sound of it you are probably getting a type mismatch, but without knowing what is not working it is almost impossible to tell.
What I think you are saying is:
1. I have a combo box which has a rowsource
2. When I make a selection an action is performed based on the selected item in the combo box
3. It worked with the first type of rowsource
4. It does not work with a new type of rowsource
So, whats the difference between the rowsources?
Give us a bit more info and we can get this one...