If the bolt will lock back on the last round fired out of a mag, then the rifle is at least full stroking.  If you needed confirm such, then single loaded round in a mag, fire the round with a empty mag still and well, and confirm that the bolt locks back on the catch.

If such is the case, then start off by checking the spent cases for signs of too much force being used to extract the spent case from the chamber.  Here, you will find that the extractor will bind the rim of the case on extraction, if the problem is that either the rifle is over functioning (bolt unlocking too soon), or could just be a case of the chamber not cleaned correctly during cleaning (chamber brush with CLP by hand used to clean the chamber) or the wrong solvents being used to not only clean the rifle, but to lube is as well.  Hence if you are not using a chamber brushing to clean the chamber and say you are using Hopper solvent to clean the chamber and bore, then the problem could be just that Hoppes leaves behind a protective resudue, and it combined with powder fouling is a quick way to chock out the rifle.


To break down cleanings (including when you pull the extractor to clean under it during B/C cleanings),
Something like Sweets to clean just the rifle bore since it does not take a lot of scrubbing and will not leave behind a sticky residue, Chamber next scrubbed with CLP, then CLP used to clean the rest of the rifle.   From here, all the fouled CLP is removed from the rifle, then the upper receiver bearing surfaces are lubed with CLP before the rifle is put into service.  

As for with the rifle correctly cleaned, including the chamber scrubbed and upper receiver bearing areas CLP lubed, if the rifle is still not extracting the spent case correctly, and no signs of the case rims bent, then suspect that the extractor spring is going weak instead.  But the fact that for the first 200 rounds, and then for the manufacturer to test the rig and it ran without problem, do suspect that the issue is a cleaning and lubing problem isntead.