just had a similar problem with 4790k+maximus vi extreme. No matter what memory I would try, It only booted with one stick on the last dimm slot. Somehow CPU got messed up and I had to RMA it. New one comes in today actually lol. Tried the CPU in a few other boards and had the same issue. Oddly it didnt have that problem before so I dont know how that happened. While waiting for the CPU to come back i sent the board to RMA too.