Plenty of mags need a bit of breaking in before they reach their full potential. This could very easily be the case here. A small amount of silicone oil down the feed tube followed by successive loading and reloading (NOT firing through the gun mind, just load the mag then manually release the catch and let the BBs come out into a bag) would very possibly help. Don't use that particular batch of BBs again though as they'll potentially be contaminated with oil.
It's worth trying before you start splashing loads of money on mags you might not need at any rate. The G&G high cap that came with my SCAR for example feeds flawlessly every time.
Also 0.3g BBs aren't really worth trying unless you're planning on running your AEG at over 350FPS. 0.28g are about the highest I would look to go with in a standard AEG to be honest.