Try hitting the lock button twice with a short delay in between each press of the button. If the bag locks that way, it might indicate an actuator that requires more current and voltage than normal to fully cycle, or it may indicate you aren't getting full current and voltage to that actuator due to poor connections. Or disconnect the other actuators so the one you're having a problem with is the only one drawing current, see if that makes any difference.
I agree with Phreakyz, start by closely inspecting, cleaning, and lubricating all the moving parts. See if there is any binding when you lock the bag manually compared to the other bag and TP. A failing actuator can cause the problem, but before replacing parts make sure it isn't just a simple binding problem with that one lock.
Jerry