Just shot my Bushmaster .308 for breakin.  Only had one problem.  After shooting 10 rounds loaded one at a time, the bolt failed to remain open on an empty mag.  The rounds cycled perfectly, just the bolt wouldn't lock back on an empty mag.  Any ideas?  

Not had this problem on my Bushy .308 but did have some feeding problems with a weak mag spring and worn out follower, how old and worn out is the mag?

Try changing out mags and see if it is the mags or the rifle.

I have three new Bushmaster 20 round magazines, and I used them all.  Failure to lock back bolt  on empty mag happened with all three.  I was shooting Federal Gold Match 168 grain.

well, I don't own a Bushy .308, but you can get this kind of problem with any manufacturer.  It sounds like:  
1)  followers need to be broken in--try pushing them all the way down repeatedly with a ruler, 20 or so times each.

2)  bolt catch is a little stiff.  Manipulate the catch a bunch while you're watching TV and ensure it's lubed.

3)  the bolt carrier/bolt/recoil spring might need to be worked in a bit.  Make sure it's lubed and go shoot.  You'll likely see your problems go away within a hundred rounds.

make sure your gas block is tightened securely to the barrel.  I have a bushy .308 and the gas block came loose and the bolt would not lock back.  It also failed to eject/feed after it warmed up (20-40 rounds).  

After taking the handguards off I saw that the gas block had been moving ever so slightly.  I tightened it down and solved the problem.

I was just about ready to send it back also.