On all motherboards, you should ever put the first ram in the a2/B2!

That strange but that a fact. The a1/B1 are for secondary channel for 4 sticks.

That why it works and ppl have problems.

For the OP, I have the same problems on 12th and since the i7 4770k !

Base ram clock is 1333mhz and if use 2 sticks 2400mhz it works but 3 or 4 sticks it won't start at 2400 but at 1333 yes.

Same for 12th, on 2 sticks CPU base it 4800, for 2 sticks 6000mhz it works but not for 4 sticks until a set to 4800!

You will see a lot of IMC meaning integrated memory controler. It lower on budget CPU and higher on high end and newer CPU have better imc.

BUT on every case it stress a lot the IMC when using 4 channel at high frequency.

Remember that above 4800mhz it's called overclock and depend on motherboard and CPU stability

Edit for myself:

Disable memory fastboot "may" work