Asrock fatal1ty x99 pro gaming memory problem

The problem that You have is often related to the chips on the memory module.
Besides Samsung and Crucial (Micron) most manufacturers are assemblers
https://www.crucial.com/support/articles-faq-memory/truth-about-memory-manufacturers
The article speaks for itself.
As long as Speed, capacity, and primary timing stay the same, these assemblers, keep using the same partnumber, and it can happen that they use different brand, and even size of chips, so one batch uses 512MB chips., of brand A, the next batch uses 1GB chips, from another brand.
An then you see, the memory is found in the system browser, becasue it just shows, the info stored on the SPD chips, on the memory modules, but Windows uses it in a different way, and sometiems you see that all memory is detected, but only half of it is usable.
With a freeware program like Thaipoon Burner you can view the content of the SPD chip, and can see if all modules, are produced in one and the same batch.
The memory controller, by the way, is NOTon the board, it's integrated in the CPU, and there is a direct link, between pins in the CPU socket, and contacts in the memory slots.
So it could also help, to check the CPU socket pins.