Subscription levels not working

The default (free) registered user level has content restriction by category. Otherwise, access to the content requires paid subscription. This doesn't appear to be working. I had a user post a question and refer to content that is supposed to be restricted. When I "view site as" a Registered User, I am able to access content that I should only be able to access if I'm subscribed to Medical Terminology.

In the Membership settings, there are only 23 subscribed users in the "Registered" subscription, when all registered users should automatically be included in that category. I don't know if the lack of default subscription is the problem.

I also don't know if this is a plugin problem, or if I've set it up wrong. I've been through the settings and the documentation and I can't see what's wrong. I'd appreciate help. Thanks!