the Jester
Legend
Do they have a worthy adversary? Maybe they need an archenemy who can throw assassins at them and so forth. Do they have vulnerable friends, lands, etc? You have to challenge them in different ways. In my Great Conflicts story hour, the pcs are currently being struck from planes away by an epic spell called scrystrike several times per day. It's pretty bad news. And some epic assassins have been dogging their heels. High level is a whole different beastie, especially if your party is smart about divinations.