Combo box won't drop down (first time)

This one comes & goes & I am convinced it's not a problem with my

Sometimes when the user mouse clicks on a combo box, it 'sticks' and
won't drop down  - with a mouse click, but it will drop when using
arrow keys.

This happens no matter which of several combo boxes is chosen. After
encoutnering the problem with the first box, all others act normally,
including subsequent attempts with the problem box.

I had this problem for a few weeks about a year ago & it went away
without any action on my part. Now it is back again.