My AR does not lock the bolt open after the last shot with 20 round magazines. I have tried brand new ones, but it still wont lock the bolt consistently after the last shot. It will lock only sometimes, but no problem with 30 rounders with green or magpul followers or even very old mags with black followers.
What could be wrong? I am using a 13.5 A1 barrel with an H buffer and a collapsible stock. But reliability is good, no jams or reliability problems. my ammo is military ammo M193, bolt assembly is new and had only 400 rounds thru it.

It could either be something wrong witht hte 20 round magazine followers, or its the bolt catch. The empty follow pushes up on the bolt catch and that is what makes the bolt hold open. Take off the upper from the lower and make sure hte bolt catch isn't sticky.

I think it is the magazine's problem as you said that the 30 rounders work fine. Check the followers on the 20s in realtion to the 30s.

I'll bet the farm its a magazine problem.