First make sure you have the latest BIOS and CHIPSET versions installed from your motherboard's Support download page.
Then do a CMOS CLEAR which will reset your BIOS back to "Default". see if Windows is showing the correct amount of cores.
If not try to tell Windows how many cores your processor has by going do this:
Changing core settings in Windows 10
- Type 'msconfig' into the Windows Search Box and hit Enter.
- Select the Boot tab and then Advanced options.
- Check the box next to Number of processors and select the number of cores you want to use (probably 1, if you are having compatibility issues) from the menu.
Normally when not checked, Windows will default to the number of processor cores automatically but sometimes it doesn't. By checking the 'Number of processors" will tell Windows how many cores your processors has.