I put a taran tactical recoil spring in mine, as a precaution. Since, I have added 14oz. to my M2 field and it locks back and cycles without issue. I am shooting 1050 to 1100fps 1(1/8) reloads.
Now, there was a lot of oil in the recoil spring tube when I first bought. If you never cleaned that out and now its turned to grime, I think that can cause friction in the system.
It is an inertia gun and that force needs to go into cycling the bolt. So, don't let the gun push you back when firing. The lighter the load, the less force you have overall, makes proper shouldering even more important.