a lot of doors have adjustment-y bits at the top. It is usually in the form of a screw that will raise or lower the door from the track for fine tuning. Some doors have it on the side of the door, and some you have to look up a little further into the track (or lift the door out of the track to get to it).

if you can work out what side of the door the sticking is coming from, you can concentrate your efforts to that side. By raising the door you should be able to prevent it sticking to the bottom of the frame. If you feel handy with a screwdriver, a half turn in any direction should be enough to tell you if it's getting better (yay) or worse (try the other direction).

Good luck!