Hello D06pblH9l, usually all cores are enabled with default BIOS settings; so, we recommend resetting the BIOS settings first. You may want to contact the motherboard manufacturer in case your system is an exception to this.
Another possibility for this behavior to happen is by having selected a number of processor cores in Windows* System Configuration (msconfig). Again, it should be using default settings such as Normal startup. Basically, it is not necessary to check mark Number of processors in the Boot tab, Advanced options.
After reviewing these settings you can run the Intel® Processor Diagnostic Tool.