Switch problem?

Hi folks. I’ve been checking in on this forum for a few weeks now and have been impressed by the knowledge and enthusiasm shown by all. I have my first question.
I have a ’92 UK spec 500SL. I seem to have a problem with the left-hand door window glass. When operating the soft-top the right-hand glass drops down like it should but the left glass doesn’t, it stays put. If I operate the left window switch separately to drop the glass then it does come back up automatically with the right one after the soft-top cycle finishes. Hope I’ve not confused anyone there!
Anyone have any ideas where the problem may be?
Best Regards