Same problem here with my new build, i installed 32gb ram 1866 and windows 8.1 shows 32 installed but only 16 usuable, after hours of reading and testing, i found one post where a guy said to turn off all power and remove 1 stick from its socket then replace, turn power back on and press MeMoK button on mobo, after saving the changes the ram set in bios to default at 1333mhz, i saved it and booted windows, then it recognized 32gb of ram and 31.9 usable, then i went back to bios and manually changed back to 1866, save and reboot windows and now have 31.9 usable running at 1866. so i think it was a speed issue when i first installed windows i had set it to 1866, but it wasnt ready for the speed up till after windows was completely installed, idk if this will help others but it worked for me. seems this is a common issue with windows 8.1 and asus motherboards.