First, just for the heck of it, please verify your operating system is 64-bit.
Okay, so you can see 8 GB on the bios screen as the pc boots up (but before Windows boots up).
Please run the memory test I indicated above first.
Then, remove one stick of memory, reboot, and note what you see.
Repeat by putting only the other stick of memory in. PLEASE keep each stick in the slot it originated it. What do you see?
Finally put both sticks back, seated firmly. What do you see now?
Sorry, but this is a process of elimination.
If all the memory tests okay, and each stick shows 4GB (or less) and both sticks show < 4GB,