gears over spin
anti reversal latch does it's job and stops them reversing, this is good, it stops the gears smashing backwards into the piston, or if they have engaged again, from double feeding with the second round be significantly underpowered.
If the motor looses power (letting go of the trigger, or the cut off lever functioning) and the gears stop just after picking up the piston/half way through the stroke there is all of the force from the spring running back through the whole gear setup, all being put on the AR latch.
As the AR latch is designed to catch the bevel gear, it does it well, but with all the extra strain from the main spring rather than just the ARL spring (the widdly little thing) the ARL cannot disengage backwards, only forwards.
At this point you try to fire again and chances are that your motor simply isn't beefy enough to get going from a semi-cocked position, it needs the run up to get going normally.
can't go backwards, ARL is engaged (very tightly) Can't go forwards, motor isn't strong enough = Jam.