Nickelodeon - Nickelodeon passed on Planet Panic

nick also has a history of turning down shows that later became big hits cough adventuretime cough
so hopefully they can keep their head held high and find a platform that knows potential when they see it

You mentioning Adventure Time reminded me of the show that got greenlit on it's place, "Fanboy & Chum Chum" It's ironic too considering that the creator of that show eventually went on to create "Glitch Techs" which has been screwed by Nick since day 0 and the only reason why it survived after it's second episode it's because it's more a Netflix show rather than a Nick toon.