I'd say that they are interchangeable, as verbs. But as nouns they differ.

"Yet again" suggest you have to try more than just once more. Igjen is just like again, it could mean once more or several more, but we don't really have a short equivalent of "yet again" in Norwegian. But it's still a valid translation. If I would have to translate "you have to try yet again" it would come back to du må prøve enda en gang.