You say Windows shows 8GB, what is been shown in the BIOS, 2 x8GB in Dual Channel? If so then all is ok it might be some bug, or if all good in BIOS but windows shows different, first try let's say some apps, first download HWINFO64 and see what is been shown in Memory Section, the second download the CPU-Z program and check the memory tab to see what is been shown! First, check this and tell me if doesn't show more than 8GB I will give you another method, OK?!
Make sure you triple-check the RAM is properly seated. If so, then refer to your Motherboard's manual to verify which slots to populate in order. Lastly, ensure your RAM sticks run at supported speeds for your Motherboard. These issues tend to be common and very fixable.