It could be several things. Weak knocker spring, mushed plate on the back of the mag. I could also be that are over filling your mags. Crummy seals which includes the valve seating to the bolt. You might want to take the valve out a little to see if your velocity increases. This also might include a weak spring supporting the brass rings that interfaces with he bolt.
Those are the correct plate but G&G has replacement parts that eliminate the need for that part altogether. I would suggest trying to simplify a overly complicated system. Any worn part could affect performance negatively but it should be very visible for you to loose 100+FPS. Have you tried chronoing the joules of heavier BBs. Usually gas gun can see some serious advantages with heavier ammo. 390FPS with .4g BBs is roughly 550FPS with .2g BBs.
http://www.airsoftoutletnw.net/index.php?page=shop.product_details&product_id=2741&flypage=flypage-ask.tpl&pop=0&option=com_virtuemart&Itemid=28
Those are the correct plate but G&G has replacement parts that eliminate the need for that part altogether. I would suggest trying to simplify a overly complicated system. Any worn part could affect performance negatively but it should be very visible for you to loose 100+FPS. Have you tried chronoing the joules of heavier BBs. Usually gas gun can see some serious advantages with heavier ammo. 390FPS with .4g BBs is roughly 550FPS with .2g BBs.
http://www.airsoftoutletnw.net/index.php?page=shop.product_details&product_id=2741&flypage=flypage-ask.tpl&pop=0&option=com_virtuemart&Itemid=28