Try updating to the latest Motherboard Non-Beta BIOS Version available and also update your AMD CHIPSET drivers from here: https://www.amd.com/en/support/chipsets/amd-socket-am4/b550
If you have the latest BIOS already installed then do a CLEAR CMOS to reset your BIOS back to factory defaults and check afterwards if Windows is showing all 6 cores.
Also in BIOS Settings check to make sure all of your Processor's core are enabled and none disabled or locked.
Also check your Windows Installation by running the following command line in a Elevated Command Prompt or Powershell. You can copy and paste it into Command Prompt or Powershell if you want: SFC /scannow
One way to verify your AMD Processor core is by downloading and installing Ryzen Master app. It will give you a lot of information about your AMD processor including if any cores are locked or not.