Scroller string moves approximately 1-2 frames in one direction and then reverses 1-2 frames and stops, does not continue to either end of the string. The Scroller sensor uses an optical sensor and a plastic disk with sections blacked out to identify position. each window passing through the sensor will be counted. if the motor is being driven, and the sensor does not see the impulses of the windows passing through the sensor, the fixture will stop and reverse the direction of the scroller to verify it was not at the end of a string. After sensing the motor movement in the opposite direction and getting no impulses from the sensor, the fixture will time out on the Calibration progress for the scroller.
Verify there is not severe scratching of the encoding wheel, and the sensor harness wires are intact. You can attempt to clean the sensor with a Q-Tip and some rubbing alcohol. If these have been confirmed, the sensor will need to be replaced.