In-cell drop-down box not working on some documents?

[Leon]

On some Excel documents the drop downlists that have been created have just
stopped working, is this a know issue, a fault with the software?

 

[Jack Schitt]

There are some versions of Excel where drop-down boxes created by data
validation do not function well on a worksheet that also has freeze-panes
enabled. Not sure whether that has been fixed in later versions. If you
consistently find the problem on worksheets with freeze-panes then this is
probably the explanation.