I have 32gb of ram installed and all sticks are shown in bios. But in both bios and windows it only shows 16gb available and the weird thing is cou-z and NZXT's can software both show 32gb of ram installed. I am using the Asus z170a and 2 8gb Corsair vengeance and 2 8gb crucial sticks all are the same speed and both sets of ram show 16gb in windows on their own.
Memory manufacturers guarantee memory in the form sold. Other combinations you decide to create have no guarantee to be compatible together. You are free to do your own compatibility testing with no guarantees.
Well i just got 16gb of new ram to add, but same brand. Had the same problem. then i remembered i messed with the timings awhile back.
I went into bios and restored original default settings, then enabled xmp. Worked, 32gb of working ram.