Hey guys, I'm having a some trouble with some of my USGI mags.  The problem is that they won't hold the bolt open after the last round is fired.  I've replaced the mag springs with Wolffe springs hoping that due to the age of the mags the springs were shot(I've had these since '87).  No joy, right off the bat, they failed to keep the bolt open.  I have had no feeding problem with these mags, they feed perfectly.  
The makes are varied, Okay, Adventure Line, Cooper.  I replaced the old black followers with new green ones a few years ago and I didn't notice this until a few months ago.
Ammo used was Q3131A, Federal XM193, Hotshot, Radway Green SS109.
Thanks for any suggestions, including the buy new mags ones.  

You wouldn't by chance be using an A1 stock would you?

What lower are you using. Loose magwell?

I just installed a Magpul stock on my EA lower.
The mags are locked in there, I have some problems with an Orlite not always locking, but all my USGI are solid.
Just handling it now, inserting a mag and pulling the charging handle back, it will lock, but when I'm shooting it won't.  Annoying.

Im fairly new to AR's.  However if there is any similarity; I had this problem with my 1911 before and it was the mag body itself.  It was a good factory mag, but for some reason it just didn't work, I had four others just like it and they all held back the slide after the last round.  Is it possible that the mag bodies are worn out? Or a certain area is beat up and needs to be bent outwards? my .02 cents.

I was having the same problem with my Colt M4gery. Sometimes the bolt would lock back on last round fired and sometimes it wouldn't. This is a fairly new rifle and my mags were also fairly new. It was kind of frustrating. I bought a Smith Enterprises carrier(for a different reason) and lo and behold I noticed that my bolt locked back every single time. I can't imagine that the carriers are any different and I don't think you can get too much better than a Colt carrier, but something is different, no problems at all now.

If the problem only originated after the installation of the Magpul stock, I think the problem is some type of limitation in the rearward travel of the buffer. I just read the installation manual on the Magpul site. It seems pretty complex, although I'm sure its easier when you actually have the parts in your hands.

I would revisit the "shimming" issue and also the unthreaded front portion of the buffer tube as possible culprits.

I asked about the A1 stock because your "symptoms" are very much like an A2 (upper) screw used in an A1 stock.

If nothing else, check and see that the mag release isn't dinged up, that's a really easily replacable part.