Not consistently redirecting to CAS login page

I have installed WP-Cassify on my multisite network, and it seems to properly direct users to my CAS login page when they haven’t yet authenticated. However, users that are already authenticated often get the local WordPress login page when trying to access admin pages and My Sites.

Am I missing something in the setup? I am not a WordPress developer, so I’m not comfortable messing with the PHP code. I’m just wondering if I’ve missed something in the setup that could be causing this.