Two suggestions :-
1. Move the problematic combo box, to a location of another control in the header section, that does 'stay put' and vice-versa i.e. that control to where the combo box was. Now open form. If the combo box stays put, it looks like its the 'location' that's causing the problem ( a new one on me, but anyway .... )
2. Delete current combo box, and place new one on the from , in the desired location ( it may help to note any related properties such as , e.g. Top and Left at the design stage) You can name it EXACTLY the same as old one, so VBA code recognises it. Good luck.
P.S. Are you sure there are no hidden controls, lurking behind it, ( maybe set to invisible ! ) ?