Is the barrel free to move - i.e. can it get stuck on a lip of paint or something? My first guess would be that it jammed and blew the recoil circuit on the mfu (seen that happen a few times now).

If the barrel moves feely, then my second guess would be a bad motor - I've also had that happen to me, remove it from the mechanism and try it on its own by holding down the limit switch and triggering the main gun.

If the motor works on its own then, the slide on the recoil mechanism could be jamming - I had one where the the slot wasn't quite formed right some how, and it would stick at 3 o'clock and 9 o'clock as the mechanism rotated.

First part of this all it to remove the unit, and as I don't have the JS2, I can't help you with that one