On 2 occasions my smaller bedroom slide, also a Schwintek, performed just as you described. Both times it stranded me on a Sunday as I was trying to get back home. They had an electrical cable that supplied the power to the motors that had a plug connector. The connector kept coming unplugged as the slide was engaged, leaving one side of the slide without power to its motor. It took 2 trips to the dealer while under warranty to replace the power line with a longer one, and a connector that was more secure. It has worked now without problems for a couple of years, but you can guarantee I am nervous every time I operate the slide. Not sure that is your problem, but the one side motor that does not have power sounds very similar.
2015 Eagle 323 LKTS
2003 Ford F250 7.3 L Diesel