Here's my problem.
My processor is i5 6600k overclocked to 4Ghz, and my mobo is MSI Z270 Gaming M7 updated to newest bios.
Widnows sees only 2 cores instead of 4. I tried msconfig, there are only 2 cores to choose.
I tried CPU-Z, it shows only 2 cores and 2 threads.
I tried doing something in bios, everything in bios is looking ok. (It shows that all cores all enabled, and shows number 4)
This is likely an issue with the BIOS. You should be contacting MSI support regarding this issue...