Hi, assuming you only worked on the belts, check that the stepped rollers are in correctly. Each belt should be tensioned by the fat part of the roller. If the rollers are both facing the same way, then one of the belts is loose.
Check that the wheel bushings on both sides of each wheel are still in place.
Check that the track is installed correctly. Sitting in the center groove of both wheels and drive sprocket.
If these suggestions do not work, send me a photo.
