Z87-K motherboard refuses to accept more than 12 GB RAM

[bootmister]

I have 4x RAM sticks and they are a mix of two different brands, which I know is not good but it works flawlessly for 12 GB RAM.

However each stick is 4 GB, so they should be 4x4=16 GB. I'm using Windows 10 64 bit and I only have 12 GB RAM in Windows.

 

[Lutfij]

...they are a mix of two different brands,...

Which is why I'm an advocate of not mixing and matching rams. Causes more trouble than good. You should be looking at a dual channel kit, preferably a 2x8GB 16GB kit or a 4x4GB 16GB kit.

Pass on the makes and model of each ram and furthermore, make sure you're on the latest BIOS update for your motherboard. Out of the two kits, retain the one that has better timings or swap all the sticks out to get a kitted ram.