Could be many reasons. Try using the Windows Update Assistant
https://support.microsoft.com/en-us/topic/windows-10-update-assistant-3550dfb2-a015-7765-12ea-fba2ac36fb3f
If your system is using WSUS to handle patches, view
https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-3-windows-as-a-service-waas-and-group-policy-administrative-templates/