Airsoft Canada - Maruzen P99 Problem

Hello everyone, this is my first post on this forum although I am by no means new to airsoft. I am posting because I am having quite a bit of trouble with my Maruzen P99. Recently for some reason when I gas it up and fire the slide does not recoil far enough back to chamber another BB. It does not do this every time I pull the trigger but, probably about 55% of the time. At first I figured it may need to be cleaned and oiled which I did; this did not help. After that I figured it had to due with the upgraded recoil spring or the metal slide/barrel I installed, but after changing it back to its stock form I still encountered the same problem. I took another look at the internals for quite some time, but was unable to find anything apparently wrong. Let me note that while I am by no means an expert when it comes to gas blowbacks I do own quite a few (Western Arms Infinity 3.9, WA Beretta perfect edition, KSC Glock 18c, KSC Glock 19, KSC USP Compact, TM P226, Maruzen P99 and an old MGC P7M13) and have decent knowledge of how they work and I would like to think if the problem was obvious I would have seen it. If any one has any insight on this matter and could post any advice on how I can repair my p99 I would really appreciate it.

-Thanks