Hmmmm, ok, last things I can think of:
Piston head, check its the correct size for the nozzle. MP9’s have 2 different size pistons/nozzles. If it’s been ever been replaced it could be the wrong one for the nozzle.
More than likely it’s a green piston cup, with a thicker walled nozzle. But they also do an orange version with a thin walled nozzle.
(you can slightly squish the thin one with your fingers, the thick one is much more difficult).
It’s 1mm difference between the 2, so can lead to issues if it’s not correct.
The only other thing I can think of for the issue of only firing 4-5 shots is the brand of BB’s you are using.
I had an issue using Excel’s in my mags, they are ever so slightly smaller so were jamming after a few shots on the metal mag lip where it joins the base of the mag. Rectified by filing down the mag lip, or using different BB’s.
I also just take any sharp edges off the rest of the mag lip to aid feeding, I have the WiiTech metal nozzle in mine and unless everything is smooth it can damage BB’s during the pick up.