G&G MP5A4 not firing every time

[uniweekendwarrior]

Hey all,recently my gearbox shell shattered and I had to replace it. Since then two magazines I used to use no longer work with the gun. I still have one hi-cap and two midcap mags that work. Now when the guns on semi it fires every shot but if I start to pull the trigger quickly it starts to miss fire. The faster I pull the trigger the more shots miss fire. When I put it on full auto it only fires about 40%. Does anyone know what the hell I've done to it? Cheers people