I had this problem too. you may want to grab some really gressy tire dressing like eagle one. The liquid one not the aerosol kind.
Roll down the window and spray some on the weather seal of the window ( the groove or 'guide' of the window) roll the window up and down and you should notice it much quicker. Do this on both edges (right and left side) of the window.
To improve it even further take of the door panel off and spray some WD-40 on the small gear of the motor (the motor it self) and do the same, roll up and down and spray as necessary.
My back windows stopped working we took of the motors thinking they were bad . But they just seized up because they got rusty. Sprayed some wd40 and let it sit for 2 days and then, helped the motor with some pliers and broke the rust. Now they work!