Is your piston locked back? You may need to release the piston as you motor won't have enough power to crank from the spring tension.
There are a few ways to do it. One is to use a thin screwdriver and release the a/r latch. Or you can stick a long rod through the nozzle and manually push the piston all the way back. But be careful and don't hold the rod in front of your face. If you let go of it, it will come shooting out.