This is your motherboard's QVL List for RAM Memory for 3200Mhz. but it is 3 years old so your RAM is not listed. But read what it says about where to install the RAM Memory on the Motherboard's slots: ROG ZENITH EXTREME Memory / Device Support | Motherboards | ASUS USA
I have attached both QVL lists for RAM Memory to this reply for you to download if interested.
EDIT: Looking at the QVL list for RAM over 3000 Mhz everyone is only supported on 4 Memory slots being populated. None for 6 slots. RAM with less than 3000 Mhz have some that are compatible with 6 Memory slots be populated.
I suggest you open a Asus "Live Chat" Support ticket and ask if your motherboard supports RAM modules of over 3000 Mhz on all 6 Memory slots. That could be why only 4 Memory slots are being recognized and not 6 Memory slots.