I had the same issue with Sulley. I think I had him at level 3 or 4 for a while and his story went stale. Then I had Roz and all of a sudden she had a quest where I had to spend a few days getting tokens to upgrade him to 8. I've been one from the school of thought that I try not to spend the magic to upgrade until I have to, but sometimes you run into big jumps like this.
If you've got the magic and tokens, it wouldn't hurt to upgrade a few characters and see if it unlocks any quests. If I recall, there didn't seem to be much going on other than the Jessie & Buzz quests for me when I was at that point too.