Have used our sliding hitch once in 3 years in a place I shouldn't have been. It is excellent piece of mind. It will be useful in spots that are unlevel and you have to turn. As others have said cap corners will have a lot to do with it.
It was probably jammed on the cotter pins. Double check your manual for the 'slide unit'; mine states the cotter pin needs to be on the outside of the rails so it is able to lock in the tow position. My guess is your mechanic forced it to lock and that was the cause of your problem.