Make sure nothing is in the bore. I've seen everything from powder residue to little shards of cardboard (no kidding!).
I also check to make sure the bolt carrier key and receiver extension castle nut are properly staked. I did have a receiver extension come loose from improper staking and/or tightening on a first range trip.
I recommend checking the part of the gas tube that comes into the receiver. One brand new AR I purchased twenty years ago or so had some grease or something inside the tube as it came from the factory. I'm not sure if the first round fired would have melted that grease and I would never have noticed it, but I cleaned it out with a pipe cleaner anyway.

