Hot streaks can't last forever. And if you're a successful recording artist with a long career, the moment inevitably comes when fans and critics feel let down by a new album. This can be because a grand experiment didn't pay off like you hoped, tastes shifted rapidly and you're suddenly dismissed as a relic from the past, you made something so bold and innovative that its genius won't be appreciated for years to come, or you simply dropped a dud due to some combination of physical and creative exhaustion, the unbearable stress of trying to top yourself, and perhaps the influence of certain chemical substances.
For truly great artists, a disappointing album can be merely a tiny speed bump on the road of a long, successful career. Bob Dylan has many albums one could safely call "disappointing," and they did little but make the follow-ups all the more impressive and interesting. We could say the same for David Bowie, Madonna, Jay-Z, Stevie Wonder, the Rolling Stones, and other acts.