death to capslock

On the one hand, the Patronus charm has the reputation of being so difficult that many adults can't master it; on the other, not only does super-speshul Harry do so at thirteen, but so do many of his friends two years later--including not only the incalculable Luna, but the never-top-of-the-class Ron, Seamus, and Ernest. Nor can we ascribe their success to Harry's wonderful teaching, for in DH Arthur Weasley and Dolores Umbridge both use the charm with no problem under stressful conditions. Producing, in both cases, fully corporeal patroni, not merely silver puffs.

In fact, we don't have a single canon instance of someone trying and ultimately failing to master the spell. Why, then, its formidable reputation? And why did Harry have such trouble learning it?

( Someone )