Was the weapon cleaned and lubricated and properly assembled/verified?
Does the ejector move freely but with reasonable spring tension present?
can you cycle the weapon by hand, with no ammunition, and feel any meaningful binding after the first cycle which sets the hammer?
do the gas pistons move smoothly and easily through their operational travel, and when removed and rolled across a true surface , do they appear straight? Are the marks they have made on the carrier face roughly comparable to one another, or does it look as though one is impacting significantly harder?
Does the extractor have full range of motion with proper spring tension, and is it's contact edge even and appear without defect?