I had a strange occurrence with my M17 - it seemed to have a miss feed every 2-3 bbs. And this was since it was new. I tried swapping frames and slides between it and my M18 and it looked like the problem was in the slide. I disassembled it and there didn't seem to be any other possibility but the nozzle. It looks like the difference between the old one and the new one was that the old one had a screw, holding the valve inside the nozzle - the new one has a sort of pin that doesn't allow for disassembly. Once replaced with the new one, it worked perfectly.
So, if you have miss feed issues with your M17-M18, you can check the nozzle to see if it is the one with the screw. If it is, buy a replacement nozzle and it should work. In the photo, you can see the pin above the gas entry port in the nozzle. If you see a screw there and the replica has feeding issues, buy a replacement nozzle. I hope this helps.