Going over Threshold to house Left drive Wheel stayed up in air and now won'tfall back down to make contack with surface of ground.. My opinion is that the drive shaft sliped out of the gear-box and is now binding against the gears and not falling back into place inside the gear-box. The wheels still turn but with the left wheel up off the ground and not getting traction anly the right wheel will move the chair and only if you turn left forward or backward while the left wheel is free spinning in the air.