I suspect it's just slightly out of spec. Either the fit between the slide/frame or the nozzle/blow back unit in the slide.
It's kind of hard to do...but you might be able to see it happen.
De-gas your mag...don't need any propellant and your face might end up close to things.
Load up a bunch of bbs...fill to capacity to check the fully compressed power of the spring (which obviously pushes really hard against the stack of BBs).
Do all of this slowly.
- rack the slide back manually, as far as it will go...the nozzle should retract, or at least retract far enough to be behind the bbs.
- let the slide go forward slowly...controlling the speed by holding the slide. Try to see exactly where the "foot" of the loading nozzle is contacting the BB in the mag (make sure you're not holding the mag up in the gun...it's just locked in there normally).
***hard to see in this pic, but you can see the foot/leg/pusher/whatever on this hicapa nozzle...should be similar in principle to your nozzle.
- The foot should contact the BB, push it out of the mag, push it up the feed ramp an into the chamber. Obviously a lot quicker in real speed.
- Then as the slide recoils (taking the nozzle with it) the leg/foot/whatever will pass over the mag and as the tip clears it's letting the next BB up into feed position.
- Repeat that process a bunch of times...slowly. Is the nozzle retacting back enough each time to pick up a bb cleanly? Is the nozzle contacting the BB on center or just below center? Is the BB hanging up on the feed ramp? Do the bb's ever flick out of the feed lips as the nozzle retracts?
i.e. is everything consistent for each load/retract cycle?
- Repeat for the whole mag of BBs, obseving each time...try some when you are pushing up on the mag...does the position of the bb change a lot?
- don't worry about the bb in the chamber from the first try...the next bb should push it down the barrel
- if the bbs are almost flicking out of the feed lips when the nozzle retracts, see if you can close the lips a bit (not always possible)...don't load so many in the mag (causes spring tension to be pretty stiff)...never release the follower of an empty mag so that it flies full tilt into the feed lips (opens them up and/or damages them)
- if the nozzle is hitting above center of the BB...it might cause the slide to ride up over the bb and either dry fire or half assed kick the bb out of the mag and cause a jam. You can try other nozzles/mags...I fixed a couple of my own by gently heating the foot/leg/whatever and bending it down a little...then filing the tip so that it picked up the BBs nicely. On most setups, having it contact the BB's at center or just below is just right.
- if the BB seems to catch on the feed ramp...take a close look to see that there aren't any corners/burrs that it's catching on.
Then you can repeat by refilling the mag with BBs and manually racking the slide back (quickly) and letting it go forward on it's own. If a pistol is working properly, it should rack/feed 100% of the time racking it manually (hold it level and "normally"...not pointed down at a garbage can or anything like that).
If none of that is conclusive...buy another mag. If that doesn't work...junk the pistol.