What to do if the first two RAM slots do not work?

Question

RAM

Updated the system. I now have an I7 8700k on a gigabyte z370xp sli. But on absolutely new hardware, the first two slots (which are closer to the process) do not work. And for this I can’t help but get a two-channel (What I just didn’t do ... (the legs are in order, everything is new and I checked it 100 times).
Sometimes you can run RAM in those slots if you kill its speed (I set 1600 MHz timings 20 20 20 40 and miraculously works)
But why is this necessary if I have 2666 by default?
Krch, please help me solve this issue! Bios, if anything, updated!