Technology

If a big chunk of ram isn't available this is usually because your system is using at as video memory, typically in the absence of a full GPU card. If you don't do a ton of gaming or video editing you can probably go into the BIOS and reduce your video memory to 1-2GB, or if you have a separate GPU then set it to as low as possible.

Hit ctrl-shift-esc to quick open tasks/performance. Are apps using a total of 8 gb? What are you running?

If you reseated the CPU and tried different RAM already it could be you have a defective motherboard. I read some other threads where that was the case.

Unlikely, but your motherboard might have an extremely low max RAM capacity.

Search for your boards specifications and check how much RAM/Memory it can suppport. If you don't know your motherboards name, open CMD and type: "wmic baseboard get product"

What other hardware do you have installed in your system?

Can you take a screenshot of the Memory tab of Resource Manager?

It looks like the BIOS is reserving the memory for something. Reserving memory is pretty normal, but 8 GB isn't. Have you tried resetting your bios to default settings.