Have you checked the O/S that your are using supports 32GB. RAM? Some versions of Windows do not support more than 16GB. RAM. These are the games that some O/S mfgs. use to exploit consumers. Your should be able to check with Microsoft on Win 8 or Win10 limits if you are using the newer O/Ss.
Below is an example of Win 7 supported RAM by version of Win 7 used:
Here are the upper RAM limits for the different editions of Windows 7:
Windows 10. Memory was used in another board (Asus) & was recognized correctly as 32gb. Currently awaiting a response from Gigabyte (their support isn’t very good). I’m leaning towards a BIOS issue & hoping a different BIOS will correct it. Possible memory controller issue w/ the CPU but I doubt that. Still scratching my head.
Unfortunately I've never enjoyed good tech support from Gigabyte even though their Ultra Durable series mobos are pretty good quality IME, other than the early AM3+ mobos which had insufficient VRM designs to power 8-core AMD processors. I agree that a BIOS update is probably required. It's not likely to be the CPU as the CPU can use whatever the mobo provides.
Having a hard time finding a different BIOS. Somewhere out there there is an F9 BIOS. Keep reading about it but haven't found it yet. And, yeah, still waiting for a reply from Gigabyte support.
I did a quick look at a couple Giga websites and only found F1 and F2 for the GA-990FXA-UD5 R5 model. Make sure to install the correct one as the model numbers and revisions are easy to mix up! Unless the Gigabyte websites are extremely out of date an F9 version would be unusual.
I'm guessing out of date. Present BIOS is F3 & not available on their sites. I read in another forum (Tom's Hardware I believe) that a Gigabyte tech gave out the F9 BIOS to someone w/ the same problem but w/ a different Gigabyte board. I'm thinking it might be a beta BIOS.
F3 would make more sense. An F9 BIOS for a different model mobo can't be used for your mobo without killing the BIOS chip and mobo operation. That's why I mentioned F9 didn't seem correct for your mobo.
Some mobo makers offer "beta BIOS" upgrades with no real support. Basically they are using their customers to test the BIOS for problems. If and when it's determined to be OK, then it is released to all. The BIOS listed for normal download are believed to be 100% correct even though that is not always the case. It's still odd that the mobo would even ship with a BIOS that only recognizes 16 GB of RAM unless someone made a coding error on the original BIOS? It's possible that the Gigabyte BIOS just don't get along well with the specific RAM that you are using? That too is usually a BIOS update to make the BIOS and RAM happy together.
Just because I am an idiot, are you sure you are using a 64bit version of Windows 10? The 32bit version is limited to 32 GB RAM.
32bit is 4GB on the nose.