Glass or mirrored sliding closet doors have a track on the bottom and top of the opening for the doors to ride on, especially the bottom track can get full of junk and prevent the door from rolling properly. So the first thing to do is clean the tracks and lubricate the wheels with a small amount of lithium grease. Second it to check the adjustment of the wheels to be sure they are not too tight or loose. If you find the wheels or carriage ore bent they will need to be replace.
Due to the way the wheels are manufactured they have a tendency of bending. The video below will show you a simple way to strengthen the door to keep this from happening.
Remember to keep the wheels of your doors lubricated for trouble free sliding.