Why could Harry and Lupin practice the patronus on the boggart?

In the Prisoner of Azkaban, Harry and Lupin could practice the patronus on the boggart, but when Harry was in the final task in the Goblet of Fire, he tried to use the patronus on the boggart, it didn't work and he then moved on to use ridikkilus.

Why could he use a patronus against the boggart, but then not in another?