If Windows 10 is not updating automatically then it is most likely due to system file(s) corruption.
To remedy this run SFC from the Adminstrator Command Prompt window as shown below. Once complete reboot and run Check for Updates within Settings, Update & security.
If this fails run DISM (Deployment Image Servicing and Management)
dism.exe /online /cleanup-image /restorehealth
When finished restart PC and Check for Updates again