In most settings—story, RPG, whatever—there tends to be at least one powerful, probably ultra-competent character. This isn’t a problem. No, really, I mean that, their existence alone is not a problem. The problem is when you’ve got some sort of Big Problem and for some completely unspecified reason, the powerful/ultra-competent/whatever character is leaving it to the main character/the PCs/whoever it is who’s taking center stage. The best way to work around this, of course, is to make sure there’s a good reason why Big and Powerful isn’t making him or herself useful. Why might that be?
Big and Powerful is busy with something else. Let’s face it, in a living world, there are probably multiple threats to population centers, if not necessarily the world as a whole, or Big and Powerful has made big and powerful enemies. Whatever’s going on, the fact remains, Big and Powerful is busy. (And hey—if Big and Powerful is busy enough with whatever’s keeping him/her out of the main conflict, and it’s a short-term issue rather than a long-term responsibility, you have a bonus chance for Main Character(s) to come in like the cavalry after finishing their thing. Can you say ego-trip?) On a more passive note, if you’ve set this up, it might be that Big and Powerful is simply stuck in one location or concentrating on one thing in order to prevent something worse from happening, and this is just a continuation of the same.
Big and Powerful isn’t suited to the job—or at least, isn’t as suited to the job as whoever has to actually do it is. Your Big and Powerful might be a good fighter, but have no subtlety whatsoever. Or lack that one particular knowledge skill that is absolutely vital for dealing with The Plot Problem. Or doesn’t have an item that is absolutely needed for this particular endeavor, and is not in a position to borrow from the Main Character(s). Granted, it’s harder to write up a generally impressive character who isn’t specialized to that whatever but still manages to be generally impressive, but it’s doable.
Big and Powerful just can’t deal with this particular problem, not necessarily on a competence level but on an emotional or a metaphysical one. I’ll grant that it’s hard to get this one to stick, since people have a tendency to devalue problems on an emotional level, but among people who accept this particular premise, it really works. Sometimes it’s attachment (love works well for this, but I’m fonder of misguided loyalty), sometimes it’s old trauma—or if you’re going metaphysics, you’ve got oaths with nasty metaphysical enforcement clauses, you’ve got a nice strong geas, you’ve got compulsions and mental rewrites and the possibilities are constrained only by the rules of the ‘verse.
Big and Powerful is actually behind it. Note that this works a lot better if you’ve had a little foreshadowing; dropping “oh yeah and I’m evil” out of nowhere, particularly if you’ve got a nice sympathetic Big and Powerful character whom your main cast honestly respects and admires, can lead to people growling about bad writing/GMing. Either way, this isn’t a plot point you want to make a habit out of, because people start expecting it and acting accordingly. But once or twice, as a gut-puncher? It hits, it hits hard, and people remember it.
Big and Powerful is out for the count—captured, badly injured and clinging to life, terribly drained from the last heroic sacrifice, killed trying to stop/mitigate the threat in question (with varying degrees of success).
Why can’t Big and Powerful take care of it? You’ve got plenty of possible reasons, and providing a good one will make for a better plot.