[e-luder]

if i may entertain you:

sounds to me like you've got a gas compression issue. It's mainly cause by the magazine rubber (the rectagular air nozzle which the gass pass through on the magazine) being worn due to the loading muzzle grinding against it. When there is too much space between the magazine rubber gas route packing and loading muzzle, gas tends to escape before it even reaches the floating valve. or at least not enough gas to cycle the gun 'crisply'

I can only venture a guess that that is what is happenning here since the problem is only inherent in one magazine. there are myriad of fixes but here is a few.

since your long magazine works, try swapping the magazine rubber with the short mag that doesn't work and see it fixes the issue. then you know you have found the issue. the you'll know what to replace or what to fix.

another way you can fix this is by shimming your bbu with duct tape. put some on top of the bbu so that you bring the bbu closer to the magazing rubber gas route thereby eliminating the uneccessary gas discharge and directing the gas directly into the Nozzle. dont put too much tape though. if you do, you cause the bbbu to be too close to the magazine rubber and wear it down even further. And also, the guide rails' path will be blocked by the bbu and the slide wont cycle at all. also the loading arm on the nozzle will jam the gun if there is too much tape.

the second thing is to disaasemble the bbu and see if anything is broken or worn.