I created a group, added users to it, put the group in an OU, and assigned a policy to it, but the policy didn’t apply. Why not?


Group Policy, amazingly, doesn’t apply to security groups. It applies only to the user or computer accounts in the Site, Domain, or OU. So, you’ll need to move the user’s account into the OU where the GPO is linked. Log back in as the user, and you’re in business.