I have the exact same problem, but only the far left Ram slot. I am using 1 stick in the right most slot. I am sending my board back in a week or two, because I am not patient enough to wait on a bios update.

I ended up buying a second board and having it overnighted (again, time constraints). To my disappointment this board is even worse than the first.
 
In this board NONE of the RAM slots allow me to successfully reach the BIOS. I tried every slot individually, with every stick of RAM I have, reseated the CPU and carefully inspected for bent pins TWICE and did all of this out of the case to avoid any shorts and EVERY time it stalls with error code b7 (Memory training). This is nuts. I was exceedingly meticulous and careful the second time around and I see no way that I did anything incorrectly or flexed/stretched the board. Is there just a huge batch of dead/defective boards or something?
 
Because I'm out of time I decided to go back to the first board which has at least two working slots. I updated the BIOS to 1.09 on the off chance that would help, but it made no difference. Still code b7 if I use either of the left slots. At this point I can't afford to buy another board because it seems like the X99 Micro's are currently as reliable as a coin toss and frankly I have no time left to finish this build.
 
What a pain in the ass.
 
 
 - Endain

Yeah, I should have specified. 2 for 2 on faulty X99 Micro's.

I got back from my trip finally, figured I should follow up.
 
I got my 3rd X99 Micro delivered while I was gone. Went through the exact same setup and testing as I went through a dozen times on the other two boards and this one FINALLY works - it gets past code b7 and actually POSTs with RAM in all 4 slots. I was able to install the rest of my system components and install Windows 7 and so far everything seems to be working flawlessly.
 
I really was starting to think I must have been doing something wrong with the first two boards as the chances of getting two defective boards (with a similar defect) seemed really slim - but that honestly appears to be the case. I just wish I could have avoided the whole hassle of having to replace the board twice. Interestingly enough this board has the exact next serial number after my second board (which was the worst of them with ALL RAM slots being defective), so it doesn't really seem like there was a bad 'batch' or anything.
 
 
 - Endain

Code b7 can be related to the VCCSA or System Agent voltage. This voltage needs to be set just right especially at the higher frequencies of 3000 MHz or more. If it is set too low or high then the system can either have memory errors or the system will fail to post all together. For the higher memory frequencies, I have found that running test 7 in Memtest86+ repeatedly will let me know if this voltage is off or if the timings are too tight. If there is an issue, test 7 will usually fail even if the other tests pass.
 
There is also the possibility of an issue with the heatsink mounting. If there is poor contact with some of the pins either due to over-tightening or uneven tightening, then there could be an issue with one or more of the memory channels.

Install one stick of memory and if you can get the system to post, disable XMP and set the memory frequency manually to 2133 MHz. Save and shutdown. Then install the other three sticks and see if the system will post with four sticks of memory.
 
If this fails, take one working stick of memory and test in each of the four individual memory slots. If the system fails to post on specific memory slots, then you may have an issue with one or more pins making poor contact with the CPU in which case re-seating might help or an issue with the board itself.