With a pistol length gas system, the gas port pressure will be high, it might benefit from a higher buffer weight.

Also, when fired, is the pistol shouldered ? It might be limp wristed in some sense.

Short barrel is going to be ammo sensitive, but lets start with making dam sure that that the key to carrier is not leaking, the gas tube is aligned with the carrier key, and don't still have a problem back at the gas block with it leaking to the barrel, or gas tube instead.

From there, double check chamber and head space, since if too tight will cause short stroking, as well as rough reamed chamber,  Hence start with spent case, since will tell you the condition of the chamber before you even start checking by eye and head space gauges.

Simply, not enough gas making it way to the gas chamber section of the bolt will cause short stroking, as well as too much gas will cause short stroking and in which a tell tail sign of that will be extractor bending the hell out of the rim on the pull.

The other ones you want to check, pull all the way back of the charging handle, make sure that the face of bolt stops 1/8"~1/4" in front of the back of ejection port window, and the last inch of pull is no greater in tension than middle of the pull.  Hence wrong buffer, wrong length spring, or airsoft receiver extension that is short depth, can cause the bolt to not come back far enough during the stroke, to be able to strip the round in the mag out of it on the feed. Next, check the back end of the disco and trigger, where the hammer tail will come down at back of stroke, for signs of denting or white line, meaning trigger tail is too long, and binding up the carrier on rearward stroke.  Yes, hammer will be cocked back on cycle, but if tail of hammer is too long, it going to bind the hell out of the B/C on back of stroke, slowing the B/C too far down.

So once any gas leaks or blockages have been solved, chamber checks out fine, distance that B/C can come back correctly, and others pointed out above,   then with the ammo that will be used in the rig since short barrel is going to be ammo sensitive as pointed out, figure out if problem is too much gas, or not enough (which you should be able to tell by just looking at the spent case/ it rim/spent primer and any marring of the spent case sides).