A lil prob with my windows...

I have a problem with my rear windows on my 1987 420SEL. When I bought the car, they worked excellent. All of a sudden, they dont open from the back anymore. The motors are excellent and they only open from the front master controls. What is the prob?? I replace the switches already in the rear and still I cant open them from the rear. I also checked the fuses and they are good. Also, one of my rear windows gets stuck sometimes when I open them?? WHY??? It would take me forever to close it!!!