I bought two, one works and one doesn't - UTG Pro AR15 Ops Ready S4 Mil-spec Stock Kit

I think this is a decent stock however one that I bought does not securely lock in place in the fully extended position. It seems secure in the farthest position but suddenly slides up a notch when firing. This is disconcerting and causes the slide not to recoil properly, jamming a round halfway into the chamber. I swapped out to the second stock purchased and everything is fine. I haven't been able to figure out why the first one fails based on simple observation. For the price I can tolerate tossing one but check yours thoroughly on the range before trusting this unit on your go-to gun