Terms and conditions are only presented when a user first registers. If the terms change, we will need to present the user with the updated terms at log in. But I can't see a way to make this happen.
After enabling Terms and Conditions (Authentication -> Required Actions), you then have to make it a required action on a per user basis (Users -> <find user> -> Details -> Required User Actions).
In theory, you could check the "Default Action" checkbox to add it as a required action for all new users, but I couldn't get that to work. This might be because we are not permitting users to self-register.
If you are onboarding users manually (ie not letting them register themselves) then you will have to add this action to each user individually as part of that process. When the user accepts the terms, the required action will be removed. When the terms change or you need the user to accept them again, you need to update each individual user again, adding the required action to each one.