I have a pretty new M1A that I drag from the safe from time to time.  Its a "NM" model.  I'm having a couple of problems with it, possibly all mag related, but I'd like to get the opinions from you guys.  At times the bolt will ride over the top round in the mag and won't pick it up (after having ejected the spent case).  Other times, the bolt won't lock back after the last round. I have tried three "very good condition" USGI HR - R mags and I have the same problem with all of them.  Any ideas?

ONly thing I can suggest is that you try to get your hands on a brand new magazine.  It doesn't have to be a 20 rounder.  Order a brand new 10 or 5 round magazine from Springfield via the internet.  When it arrives, load it up and try it.  I'd be willing to bet that your used magazines aren't really in that "good" of codition.  Perhaps the springs are bad in the magazines.  If the new mags work, then you'll know for sure that the problem wasn't with the gun itself.

Buy them from CDNN Investments $19.99 compared to over $30 at Springfield Armory and Fulton for the five and ten rounders.
They are the same Mags too,CMI.

I have the same problem with my M1A thermolds it is because the mag does not seat well in the rifle or the mag sits too low in the gun. CDNN has some good prices but Cole distributing has some even better prices and don't worry about the collector value of the mag if you own an M1A buy the mags and just shoot the fucker.

I had a similar problem with a rear lugged Springfield receiver (SN #100,XXX).The problem was (is) that the slot in the bottom-front of the receiver for the op rod spring guide was cut too shallow. The back of the guide is what locks onto the front of the magazine and cams it up into the receiver until the mag contacts the "stops" and holds it there. On mine, the mag wouldn't cam all the way up before the guide was stopped by the shallow notch.Rather than even consider modifying the receiver, I cut and filed the guide so that the guide mag contact point would lift the mag all the way up and rest against the "stops".

you might want to check you bolt hold open tab... especially where the mag follower hits it.. it could be worn there... the edge where the follower hits it should be perfectly straight with no concave areas.. a buddy of mine had this same problem.. and I looked at his bolt hold open tab and sure enough.. it was worn.. but then it had been on his weapon for the last 4 yrs and the weapon had seen about 2500rnds a year through it in HP matches...