I have a Bushmaster XM with the 14" inch barrel and  the 2 1/2" permanently attached flash suppressor (not the one listed in the for sale section).  I cocked the rifle, fired a round, and then tried to fire a second, and it only clicked.  It hadn't chambered a round.  It would blow back after the first shot but wouldn't chamber the next round on its way back into resting position.  It would simply slide over the top of the next round in the magazine, sometimes scratching the case slightly, sometimes not touching it at all.  I could cock the rifle slowly manually after each shot, watching to make sure it caught the next round, then letting the bolt slap shut the rest of the way, and it would work that way and chamber a round that would fire.  But I could never get it to catch the round itself after firing a shot.  I tried everything.  I made sure the magazines were seated properly.  I tried new and used magazines and different ammo including  Remington .223 straight out of the box.  Same problem.  

Any help would be appreciated.

Does the BCG lock back when you only use one round in the mag. ? If you haven't tested it that way you should & use diff. mags for the testing also , to see if one may be giving you a problem .
If the bolt catch doesn't lock the BCG back after last round , its a gas or buffer spring problem .( not to mention other things ,but that's a good start.)

Some thing for you to check ,any way.

I have used new and used mags.  The BCG does lock back.

I'm thinking gas...
Remove the Bolt Carrier Group (BCG) from the rifle. Extend the bolt while it's still setting in the Bolt Carrier, and set it face down on a flat surface. If the carrier slides downward, the bolt rings need to be replaced.