If your gears don't move due to jamming, I would also try isolating the cause of your gears not moving (remove the spring first)
Does your spur gear spin freely with the sector gear moving with the teeth on the piston? If not, you should look at whether you've installed them correctly.
If they interlink OK, see if the bevel gear causes a jam there. As Dan said, your anti-reversal latch and/or its spring may have been installed improperly.