1. I had caught Cobalion as soon as it was available, but I didn't catch Virizion (Virizion appeared normally). After Terrakion didn't appear, I did go back and catch Virizion, but it hasn't helped.
2. I've walked all around Route 22 and reentered the area multiple times to no avail.
It's possible that I skipped something, but I can't think of what it could be.
I've just gone back to look through the tutorial, and if I'v missed something, I didn't find it. Virizion and Cobalion did appear normally though, and this is the first time I've had a problem with my game.