Either the BB's are just getting stuck, in which case I'd stack them as carefully as possible or consider trying a different brand of BB's. Some have different tolerances to others and some like to stick just a little bit- or there's some dirt/damage on the follower/follower track, so maybe taking a look and cleaning it would help.
Alternatively, it could be this.
The PMX/MP9 magazines have a strange follower design, in which you pull down the follower and it will lock itself for you, you should see a small tab at the bottom of the follower pop out. Once the magazine is filled, you have to push in that button again to release the follower otherwise it'll feed one or two BB's then stop, as the BB's aren't being pushed up. It's caught me out so many times on my MP9 and its a fair simple, yet easy-to-forget thing to do.