Most likely dirt or dead bucking: clean out barrel, check bucking isn't contaminated and is clean, check the BBs aren't picking up dirt from inside the mags.
Ohhhhh! I've just twigged, this is the angry young chap who used Just Cos insurance to buy his RIFs, instead of UKARA. I'm not sure that he's ever actually played airsoft. Given his rageaholism (he's been sending me "U WOT M8?" messages on the side), that's probably for the best. *PLONK*ed.