Personally (and fortunately) I had no personal issues with JPA. I guess I was lucky.
However, in my previous life at 'Sleaford Tech' the trainee NCA under my command had a whole host of problems, and JPA was the bane of my life as I attempted to get them paid properly (or just paid, in some cases). The staff in SHQ were very helpful but had no access rights to the system to correct errors, etc. I eventually resorted to writing personally to AMP, and hand delivered the letter at the CASWO conference.
Only then was the main error identified. It transpired that when the youngsters were 'plugged in' to JPA someone had ommitted to tick the box marked 'Trainee Aircrew'. Once corrected things improved slightly, but there were still enormous problems getting back pay sorted, etc.
I really hope they get their ducks in a row, 'cos it was causing major morale issues for the NCA trainees. P
ing off new recruits is not a great retention or recruiting tool!