[Dire_Wolf]

hmm... dunno what it could be then... I have a few mags with the tabs behind the bolt catch parts broken off, but they all work just fine in my scar... maybe check for the top plate being level? if anything got bent upwards or downwards during installation..

i'd agree with the valve getting stuck, i'd vent the entire mag a couple times outside of the gun (or take the lower off, put the mag in, pull trigger) to get rid of sealant crap...