If the coverage path is setup properly it should go to cover. What does the cover path look like? THere is a DND flag on the cover path. I've had issues with DND in the past. Don't like it for this very reason and use SAC instead.
Cover
Must be assigned before a user can activate Do Not Disturb (Hospitality
Services), Send All Calls (SAC), or Go to Cover features. Allows a calling
user, when calling to another internal extension, to redirect a call
immediately to coverage by pressing a Go to Cover button. Allows a
principal temporarily to direct all incoming calls to coverage, regardless of
the other assigned coverage criteria by pressing the Send All Calls (or
Do Not Disturb) button. Send All Calls also allows covering users to
temporarily remove their telephones from the coverage path.