I just added 2x8 gb of g.skill rip jaws V DDR4 2400 to my rig. So now I have 32gb total 4x8. But all 4 are only running at 2133 and won't change to 2400. I have a maximus viii hero alpha motherboard and a i7-6700k cpu.
You are reading the boot up value of the RAM, not the actual/current operating value. Look somewhere else like CPUz or ADAI64 and confirm it is running ~1200MHz or DDR4-2400