Problem is, your door is lower at one end than the other. In the worst case, the actual track has settled (possibly was installed on top of carpet) is not level. You could simply lay a spirit level on top of the track to check it before you start...
If the track is not level, remove doors and relay track (not a simple job). If the track is level, lay the spirit levl along the top of the door to see if they are level on their rollers. If not, the adjusting screws (at least here in the U.S.) for raising or lowering is in the middle of the roller wheel. Has to be done from the inside of the closet, so get a flash light (oh... excuse me... a torch) and either a screw driver or an allen wrench to adjust the setting screw to level the door...