i doubt its a lower issue - but if you've got another, or can access one thru a friend - its always worth a swap to see if the issue goes away.
it might help recording the shots to see if your BCG is cycling fully, but it sounds like it is at least some of the time.
without knowing what company's mags you're using, we wont be able to know if they're on the good or bad list. that info will be useful.
it does appear by looking at their site that they only sell 16" uppers, and the specs show carbine gas system, but they do not give their port dimension.
i'd probably suggest as noted to dissassemble the upper, check that the gas block is correctly aligned, and while you're there to get a port dimension.
you're looking for a good clean carbon ring completely around the port hole
here's some pics from the web showing what a good & bad alignment looks like based on the carbon ring
good carbon ring showing solid alignment.
if th carbon ring looks like the one on the right - or even worse where it intersects more of the hole - you've got an alighment issue to address
i would also look for any signs of leaks around the gas block. you'll see carbon coming out if there is.. its pretty obivous when you look for it
also note - most low profile gas blocks should have a small gap between the block itself and the shoulder on the journal closest to the chamber. the journal and gas port location are usually cut to accept a traditional plastic handguard with na a1/a2 style front sight post - and those have a small peice of metal there to hold the front of the handguard in place. if your gas block is touching the shoulder - its probably blocking at least SOME of the port.
quick pic again from the web showing the small gap there.
again these are just some examples of things to look at while you're in there.
HTH