Just resolved an issue that sounds exactly like what you are experiencing. I swapped out some watercooling fittings on my CPU loop and while doing so I thought that I would replace the TIM and check/clean my CPU waterblock while I was at it. When I started it back up the BIOS showed that I had all 8 of my DIMMS populated but all of my programs only showed 24GB's of RAM. I when over all sorts of testing of my RAM and the DIMM slots and found that I had two DIMMS that would not show the RAM that was populated in those slots in the total memory. Turns out, and to make long story short...sorry about that, try reseating your CPU....take it all of the way out of the socket and put it back in. When you put your CPU heatsink back on make sure that you do not over-tighten it
as doing so can cause problems with the pins not pairing up properly on the CPU.
Hope this resolves your issue as well.
BTW, this happened to me and a friend of mine and reseating the CPU in the socket corrected the problem for us both.