IF the regulator is not kaput, sometimes the motor could just "stick".
Try this the next time it doesn't work...Now, before you start thinking I must be crazy...No I'm NOT. I learnt this from a BMW book of tips and tricks, and it has worked for me on my previous E34. It has saved me the replacement cost of a regulator over several years just using this technique until I finally sold the car!
Try to locate where the motor would be located behind the door panel.
With a clenched fist, use the bottom fleshy part of your fist, and give the spot a good firm punch. Try the switch, if it doesn't work, try the punch again.
Good Luck!