Unfortunately, if the game doesn't think you have been kicked out of the castle, I am pretty sure it will not trigger a subsequent required quest.
Try resetting Janus' position, as described in
Reviving dead NPCs (search for 'Janus53'). Some characters who show up in more than one place do not follow the described data format, but Janus seems to (I don't have Divinity installed to check, though).
