In OS X Yosemite, FileVault, "Enable Users" is not available. Please advise

My user account is an Admin account.

Even after unlocking the preference pane for FileVault, "Enable Users" is not an option. Every help page or web search I've found has shown the "Enable Users" button in a screenshot of the prefs pane.

However, other users can still log in from the login screen!

Can anyone explain this behavior? I need to know what's up so I can administer my office's population of 20 or so Mac users.

Screenshot: