Have you tried using the S&B with different magazines?
Exactly where does the round that doesn't chamber end up? Is it still in the magazine? Does it end up wedged against the feed ramp? Does it end up wedged in the chamber? Does it end up part, but not all the way in the chamber?
Are you running lots, and lots, and lots (you get the picture) of oil on the BCG???
What are the tips of these boolits like???
I'm gonna guess that all your other ammo is right on the edge of not working also. The S&B is probably giving you information about the overall situation of lubrication, gas system integrity, buffer spring condition, etc.
It's most likely -- in this order -- the magazine, insufficient lubrication, loose gas key on carrier, misalignment of gas tube to gas key, out of spec buffer spring, pinched gas tube at front sight and/or barrel nut.
You don't mention the brand of AR. That might also be helpful diagnostically, since some brands have patterns of failure. It is also important to know the type of rifle: is it a 20", full length gas system, a carbine with short gas system, or a middy with the in-between length (mostly rock rivers).
Answer these questions and we'll go from there.
It's most likely a simple thing, so hang in there.