I had this problem with a P9X79 Deluxe. After removing and reinserting 14 different RAM sticks, I finally gave up and sent it back. When I got my replacement, the same thing happened.
I was just about to RMA this one and, out of frustration, I pushed REALLY hard on the RAM. I heard a click. It turns out that the side of the slot without a latch will actually click when the memory is properly seated. Even though it looks like it's properly seated, if you didn't hear that click, it's not. I installed all 8 sticks, making sure to properly press in the non-latch side, and it booted right away.
wow! Thats it?! Damn my luck. I really thought the memory was in all the way! Thanks for he reply! Out of curiosity, whats your build like?