Bathroom shower doors that do not roll smoothly should be repaired as quickly as possible. If the door drags on the track, it can cause permanent damage both to the track and to the top rollers.
Check the Top Rollers
Inspect the top of the door that is not rolling smoothly. Often a roller will have has popped off the track and simply needs to be replaced. You can gently lift the door to place the roller on the track.
Remove the Doors
If you can see that the rollers are on the track correctly, remove the doors to check the rollers. The small plastic guide can be removed by unscrewing the screw in the center of the guide.
Check the Rollers
Once you have removed the doors, check the rollers at the bottom. Spray a light coat of silicone on each roller. Test the rollers by spinning them. If they do not move freely, you will need to replace them. Replacement rollers can be found at a local hardware store. Use one of the old rollers to match to what you find at the hardware store.