Correct me if I'm wrong, but the symptoms he's describing do not sound sear related to me at all. If the hammer was being blown back far enough to be caught by the sear and the sear wasn't catching it properly then the second shot/cycle would hit the valve just as hard as the first...starting the firing cycle all over again resulting in extremely fast full auto firing/ball chopping. At no point would the gun end up just in the forward un-cocked state after 1 to 2 firing cycles like he's describing, it would keep cycling until physically stopped or until out of air.
The symptoms being described seem to me to be inherent of the hammer not getting blown back hard enough/far enough for the sear to catch it...ruling out the sear as being the problem. This is likely either a valve or pressure problem with not enough force being supplied to fully re-cock the hammer, or a friction problem of too much friction so that the hammer slows down too quickly and doesn't make it all the way back to the cocked position.