Tighten the brackets that hold the track in place. This may be the culprit behind rollers that become misaligned. If the track is not stable, the rollers will fall off it. The brackets attach to the metal track and then anchor to the wall with bolts. Tighten each bolt with a wrench to pull the track back into alignment.
Wipe down both the track and the wheels with a household cleaner to remove dirt and debris that may interfere with movement.
Lift the door until the rollers hover over the track and then set it down. Repeat the process on the other side of the door if necessary.