SithNai

Could be the ram and board are not getting along with each other. Try 1 stick and see how much it reserves. If there is any memory allocation in the bios for onboard graphic, try disabling that or set the ram size to 0.
And update the motherboard's bios.

Then try reinstalling the OS.

It's okay. Anyway, if you look at his task manager. 16GB is detected by the board, but windows is reserving 8GB, leaving the remaining 8GB for the end user.

Physical ram size have nothing to do with dual channel. OP's new board only has 2 slots so those are the only slots that can be used. It may be the memory controller in the CPU, try reinstall the CPU and CPU cooler, or reinstall a clean version windows.

Sorry, i thought he had 4 dimm slots.

Uh.. Thx.. This might help u: 
 

U are using a dual channel kit... U might have placed it wrong... check the manual where to place them correctly
Or use a XMP Profile in u'r bios