As I recall, that will open up either when you confine the Draconic Crusader, or once she reveals herself after Queen Xenon's missions.
She might be a late bloomer.
Unfortunately, a hard drive crash and backup failure has left me scrambling around for old copies of my source files, but there shouldn't be a way to lock yourself out of completing the dragon sanctuary.
But if she doesn't come around by the time you're trucking around with your 5th/6th regular party member, then something may well be wrong.