Did you lube it properly?

 

Ok, so, do you have more than one mag that this is happening with? Is the mag loaded all the way and filled with gas? Is this semi or burst? Is the selector all the way to either position? Is the slide catch staying down on its own? Also, how long have you had the gun? Was it working normally for a bit, and then stopped?

 

ive owned this gun for a month, and it always did tht and it locks back on all types of fire, and i own one mag

 

I'm not sure about the kwa m9 however in my 1911 eventually the two screws holding the hop-up chamber together will loosen just enough to prevent the outer barrel from moving and thus stopping the slide. check to make sure your hop-up chamber screws are very tight

 

Probably the mag.

Does it still lock when you hold the slide lock down and fire?

If so, then for some reason the lock catch on the mag is stuck in the up position.

 

And THEN lube it well, after the above suggestions