Steve - a little clarification may help us here. Your DVA serial number indicates to me that it's a new rifle, correct? Since you're in San Diego like I am, you have 10 round mags. Do you have more than one? If so, is this happening on the same mag or on different ones? From your description, it appears to be a failure to load a round, not necessarily a failure to eject FTE, UNLESS there is a spent casing causing the bolt not to close.
Let's presume it isn't an ejection issue. Next let's make sure that when you're loading the mag, you are allowing the rounds to seat in their natural offset, similar to a double stack magazine in a Glock pistol. If you don't have this offset, it's been known to cause feeding problems. It may be worth watching the loading video stickied in the very first message on this forum:
http://smith-wessonforum.com/smith-wesson-m-p-15-22/125703-m-p-15-22-instructional-video-series.html He gives some hints on loading to avoid problems.
Next let's look at the ammo. What are you using when this is happening, and is it happening only on a certain brand of ammo?
I'll be at the American Shooting range on Ruffin Rd. Tuesday about 4:00pm if you want to meet and compare notes.
John