"I'm not going to lie to you, I think there are problems," he said.
"Millions . . . watch the show every week and fall in love with a singer, then you don't hear much from them. We haven't had a breakout success like Kelly Clarkson or Carrie Underwood," added the Maroon 5 frontman. "[The show is] very successful, but it's got a ways to go."
Tessanne Chin, winner of The Voice's fifth season, and Josh Kaufman, winner of the sixth season, are still waiting on the release of their albums. Cassadee Pope, the most successful Voice winner, sold only 175,000 copies of her country album, Frame by Frame.
Notable deaths of 2014 [PHOTOS]