do both drive wheels have the rubber intact? To me it sounds like one is missing. You could also have a motor that is a little lazy or debris on the magnet clutch in the gearbox.
If the rubber “tracks” on the drive wheels are intact, I would make it run for a while with the drive wheels off the ground and make it go thru a few “forward + 99” commands, and see if the clears the clutch and gets the gearbox to move more freely Aside from that a complete teardown and cleaning of the gear box, which is very involved.