I'm having an odd problem with two of my AR's.  In both cases, the bolt catch will not hold the bolt open.  With an empty mag, still will not hold polt.  It appears the bolt is not being pulled far enough to the rear to catch on the bolt.  One upper is an early M16 slabside upper.  The other is an A1 upper.  I have alternated lowers among a DSA, Rock River, and Oly with RRA parts kit.  I have exchanged bolts (new Colt M16 and AR15), and even charging handles.  The bolt catches all function smoothly.  I'm stumped, unless there's something specific with the older models.  

I have two other AR's (A2's) that do not exhibit this problem.  

Any thoughts would be appraciated.

Thanks.

Somebody's got a case of the Mondays...

worn mag followers?

What type stocks and buffers? A1 stocks used with A2 top screws can cause this problem.
Also there are some short bolt catches out that are not long enough to be raised by the mag follower in some mags.

In both cases, stock is M16/A1 w no trap door.  I think I may have used A2 top screws.  That would possibly explain the failure of the bolt to go far enough to the rear.  I'll check it out tonight.  These are both very nice rifles and the lowers have little use, so parts wear was an unlikely possibliity.

Thanks for the help.  I'll know in a few hours.

Thanks for the advice.  When I got home last night, I loosened one screw and the bolt easily moved back fully.  So now I have two fully functioning AR's again.  I was aware there were two lengths of upper screw, but did not think of interference since they assembled fine.