i honestly have no clue whats wrong. everything seems to be put together correctly. i can only suggest tightening your scs, but you said the bars wont spin then
i wear one for hockey. i got a pretty bad concussion a few years ago and mouth guard help reduce the vibrations. i suggest doing what madis said. and getting one from the dentist. thats what i have
id practice feeble stalls. its a good way to get the feel of the trick and practice getting in the right position, without the risk of falling. once you get confident with those, try actually sliding