I don't know if I am doing this procedure whole wrong, but when I try to tighten my back tire, the nuts just keep spinning and not tightening. I made sure to hold the opposite still screw it in, but the nuts just spin rather than tighten. Is it something wrong with my bike or are the nuts faulty?
Sounds like the threads on the nuts and/or the axle are stripped.
Check there is an equal amount of length on each side of the axle,coz if not you will have trouble getting the wheel on. If the threads are stripped on the axle or the nut,then you will have to replace either one!either take it to your lbs or look at videos on how overhaul wheel bearings,as they will show you the procedure for removing/installing the axle.