Diagnosis
The trolley is likely worn or the rail is misaligned. The trolley has internal nylon gears that mesh with the screw. If worn or cracked, it can jump track.
How to Fix It
First, check rail alignment. The rail must be straight and the brackets secure. If aligned, replace the trolley, part #GTR-TR. Disconnect the door. Manually thread the old trolley off the screw drive. Thread the new trolley on. Ensure it moves smoothly the full length. Reconnect the door and reprogram the limits.
If That Didn't Work
If the new trolley also derails, the screw drive itself may be bent or damaged. Inspect it for any warps or kinks. A bent rail section must be replaced.
Heads Up
A derailed trolley under tension is dangerous. Always disconnect the door via the red cord before handling the trolley or rail.