Be sure to scroll down... there may be more than one question on this page!
Since there are different hardware styles for the rollers on pocket doors, it is hard to say how you would repair it. The problem could be loosened or stripped screws holding the roller(s) to the door. Some types of rollers are made of two parts... a base attached to the door and the roller itself... which have either separated from each other or gone out of adjustment. You should compare the functioning and malfunctioning rollers to decide which problem you have.
Either repair requires you to be able to get your fingers or a tool to the affected parts to make the proper repair or adjustment. You may be able to remove some of the molding around the frame to increase your access. The difficulty of getting the molding off properly and without damage increases if the frame is painted, because the order of assembly is not as obvious as if it was a stained finish.
If your attempts at repair through the frame are stymied by limited access, your only recourse is to cut a hole into the wall to allow adjustment to the hardware within the wall. I usually drill a 4" hole at the proper height on the side of the door that allows me the best view of the track. However, if one side is wallpapered I would suggest making the hole on the other side to save the paper... but only if I am confident the repair can be done that way. Again, I can't tell you this... you have to examine the hardware and make this determination yourself.
There is no magic bullet in pocket door repair. Pocket doors are meant to last a lifetime without repair, but poorly designed hardware and installation errors sometimes cause them to fail, leaving us to do the best repair we can.
I have a more thorough article on pocket door problems here: