I've had this problem a while and have finally figured out what it was but need to know how to fix it.
I was getting a lot of light primer strikes and swapped out the upper internals to diagnose. Doing so led me to find that I'm firing somewhat out of battery - without the bolt completely forward. (I don't think this is normal, is it?)
It seems the hammer is falling, pushing the bolt forward some more and not striking the firing pin enough to ignite (thank goodness).
If I use the forward-assist before each shot it cycles fine except for bolt lock-up.
This happens with all bolt/carrier combinations I could swap out.
Where's the problem located and the likely fix?
Hammer binding up on the firing pin collar during the forward cycle.
Barrel receiver extension out of index with the bolt.
Read the FAQ's to find the way to resolve the problem.
Are the FAQ's too old? They're trying to link to messages from 2003 and just return the current forum.
To check for hammer-firing pin binding, hold the trigger back and pull the charging handle all the way back, then slowly ride the charging handle forward. The carrier should only bind up when the bolt enters the receiver extension lugs (just before the bolt turns/ locks).
If the carrier stops about half way forward, the hammer is binding up on the firing pin collar, and you will need to relief the top L cut of the hammer's firing contact pad to allow the firing pin collar to slip past the hammer during the re-charging cycle.
Also, since this is a pistol, and does not use any type of buffer, the next item that comes to mind is does the recoil spring have enough tension to fully charge the weapon (strip a round and lock home), but not too much that it is causing carrier bounce with a too tight chamber (head space) to not allow the carrier to re-lock back home after the bounce.
As you can tell, novels can be written on trouble shooting the pistols since there are more than a few designs. Due to this, it's just way faster to have the thing in smith’s hands and allow him to correct the pistol as a Whole unit since they can be beyond temperamental.
The problem is finding a smith that knows the pistols and can trouble shoot them quickly. If all else fails post over in your Hometown forum to see if anyone around you can give the pistol a once over. Tweak may be from your part of the world, so you may want to butter him up and see if he can take a look at it (helps to snivel and plead with big alligator tears in you eyes if your trying to get him to do it for free).
As for myself, I just got home and am on Holiday (trying to complete my own shit that has been sitting on the bench for the last year when I was on the road).