I would recommend finding a buddy who has a stronger motor and battery, and trying that. If that still doesn't work you'll need to open it up and see why its not working.
Generally when what your describing happens to me, I try another battery, then confirm my motor is connected red to red and black to black, and try what Coachster is saying and release the anti-reversal latch or release the piston manually. If that fails open it up. What your describing usually happens when your piston is stuck all the way back as Coachster has described. Can you see what position your piston is in?
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Even sung, or spoke, or screamed, they will betray what they had meant.