If it's anything like the SW-7, the grease in the worm gears in both trucks is an issue. That brown clunky grease needs to be removed and Lithium grease needs to be used.
Make sure they have the shaft from the truck to the horizontal motor is installed correctly. Make sure the truck assemblies are assembled correctly and the worm gear is not loose. There's not much to see popping the hood. Most of the business end is in the two trucks. Check those areas first.
