Really glad to hear your slideout working again. There is a plug-in electrical connector right beside the motor that may be loose but doubt if a bang on the wall would fix it - more likely the opposite, it would fall out. Perhaps the gearbox was "jammed" (mine was seized, I think, from water ingress during winter storage when Ontario had the worst winter in recent history) and the thump freed it. Have the dealer check for signs of rusting around the motor gearbox.
I should clarify, I really like my Lacrosse and have had new trailers before and none are trouble-free - it's just the nature of the industry. They are very light on manufacturing documentation and it seems too much is left up to the individual building the trailer. However, once the initial bugs are out, I've usually had a good experience.