This is gonna sound silly...but trust me. I had a E34 previously and it worked for my front passenger window every now and then when it decides to stick.
Locate roughly the whereabouts of the regulator behind the door panel.
Give it a hard punch with the soft bottom of your fist.
Sometimes it sticks, and this will kinda "release" it of that sticking point.
If it doesn't work...you know you need a new regulator! :lol:
Good Luck! And let us know how it goes.