I have a Wilson Combat AR, and the bolt won't stay open after the last shot.....What give? I used the USGI 20rd mags.

For some reason, the bolt catch isn't popping up in front of the bolt.  First thing to do is see if the bolt carrier goes back far enough.  With an empty mag inserted, pull the charging handle back all the way and look in the ejection port.  You should see the mag follower pushing up on the bolt catch.   If the carrier doesn't go back far enough, it's probably a spring/buffer problem.  If the catch doesn't come up, it's probably just a little stiff from being new.  If it's stuck down, you need to work it by hand a few dozen times and lube it.

Most likely, it's just new & needs exercise.

edit:  eventually, I'll learn to spell.

Thanks Norm.

Glad to. I had the same problem on my TanneryShop 80% AR.  The catch would work by hand but the mags didn't have quite enough poop to lift the catch during the short time the bolt was back.  A lot of hand wiggling to smooth out the slot, a little lube, and a bunch of firing and now it works fine.