I had this same issue late last year. I performed the synchronization as outlined in Schwintek manual. After many tries I called the factory rep. He said do the following:
"Make sure the slide is all the way in. Hold extend button until slide goes out ~6". Then retract the slide fully. Do not let go of the button until the slide is fully retracted and you count to at least 10 seconds after motor humming stops. Then let go and repeat the above steps 3 times. This should solve the problem."
Every time you retract or extend a slide ALWAYS hold the button for 10 seconds after motor quits to ensure a good synchronization of the motors.
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