You need to go through a process of elimiation.
1) Take out the spring, piston, cylinder/head, tappet and AR latch, screw back together. Use a small screw driver or similar to spin the gears (go in through the cylinder window and spin the sector gear). They should spin freely and keep spinning after you flick them with your screw driver. If they dont move or are hard to move or dont spin freely - your shimming is too tight.
2) Check your motor alignment in relation to the bevel gear and the spacing between the two - this is really important to get right. Test this with just the gears and the motor and grip installed. A "screechy" gearbox when fired on full auto will reveal issues with the motor alignment and shimming.
If all is ok after doing those, then move on to testing with the piston etc installed.
There are loads of good tutorials on youtube for shimming etc: https://www.youtube.com/user/AirsoftingTutorials/videos