How many rounds have you put through it? Number your mags and see if it happens more with one than another. As said. Lube it well.
Mine was iffy about locking back till about 200 rounds.
I have a lower that's known to be touchy with PMAGS and I run a 16" rifle length system which makes it a really soft shooter. I haven't run many different types of ammo, but I don't seem to have any issues anymore. Just took some breaking in.