Verify that three wires are connected to the drive.
To access the drive you will need to remove the plastic enclosure from around the top frame. Use the pedal to elevate the table. Unplug the table. There are two screws at the head end, near the arm rests, two screws at the foot end of the plastic, and one on each side. Pull the plastic down and away to free it from the arm rests if you want maximum visibility.
Verify that the two yellow wires and the black/white hash are connected securely to the drive.
If so, then check continuity of the three wires back to their source. One yellow routes to the reed switch and the other yellow joins the other reed switch lead along with the white/black from the horizontal switch located on the control box. The black/white connects to the horizontal switch.
Verify function of the horizontal switch on the control box. Use a meter to verify continuity. Horizontal ON occurs when contacts are closed.