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.
So I managed to figure out what went wrong with the dragon castle, but apparently, releasing the Harpies is permanently missable. I can't talk to the wind spirit or get the crystal, even if I reload the save I have that's closest to the point in the guide where it says to go there. The only difference between this save and the guide is that I rested with the mermaids before going to the cliffs.