Motherboard detects my memory just fine but Windows doesn't

[XtremeAero426]

My motherboard detects both dimms just fine:

But Windows only detects the 8GB stick:

And even weirder is that CPU-Z only detects the 4GB stick:

How do I fix this?

Here's my system specs:

Intel 4690k @stock
Asus ROG Maximus VII Gene
PNY 1x8GB XLR8 DDR3
ADATA 1x4GB VLP-R DDR3 (Windows isn't detecting this one)
MSI RX-480

 

[SkyNetRising]

Try different ram slot configurations:
using single 4gb module in all available slots (to determine if there's any problem with particular module or some particular ram slot);
using single 8gb module in all available slots;
using both modules in various slot configurations.

 

[The Paladin]

According to your manual http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/MAXIMUS-VII-HERO/E9799_MAXIMUS_VII_HERO_V2_WEB.pdf?_ga=2.257538120.271517015.1521801534-1793528967.1507551773

There are special layout for DDR3 or DDr2 slots only rules, Cas speeds being different, and if ram is designed for single channel or dual channel, as I read the last copied image below, your issue is that the motherboard can only handle 1 single channel ram stick or 2 dual channel ramsticks, and since you cannot dual channel with mismatching ram sticks it only recognizes one of the two

Get another identical ram stick to your 4gb or 8gb. then set them up as dual channel ram sticks.
PS: I would look up each ramstick 4 and 8 model type to make sure those ramstick types can actually run in dual channel and are not cheap low end single sided only ramsticks, and if it is the case then you will have to be happy with 1x8gb or exchange both for a pair of dual ramsticks

Ref: