Could be a few things:
Magazine not sitting quite right causing a slight misalignment with the feed tubes : wobble the mag while firing semi and see if it makes it better.
Gunk in the magazine (or chinese lube!): covers the bb in lube, which means that the hop does not grip it.
Could be gunk int he gearbox and on the air nozzle as well causing issues.
Could be the ammo type.
Basically put a 1000 rounds through it on full auto to wear it in a little and it should settle down, wear in a bit and you will be fine.