The most common cause of this is limescale which has either caused the wheel to fail, or there is a build-up in the frame/door runner. In the first instance, check both and clean thoroughly to see if they free up. If this does not fix the issue you can purchase replacement wheels from our store here 

Tip! Use Vaseline or some water repellent grease to help the wheel run much more smoothly which in turn will help prolong their life.