Hello Michael,
I’ve checked the Force Login plugin and it appears that it may be adding no-cache headers to prevent caching.
It is not advisable to cache pages to logged-in users. The reason for this is because some secure content may be displayed to the other users that should not have access to this.
Also, please share some more information on what you want to achieve, what do you expect to be cached, and do you want pages also cached for logged in users?
Thanks!