"Oh no! None of your pets are eligible for battle. You might want to make sure that they're well fed and happy! "

Well, my battle pet, Nyuubi, is unhungry, and very happy. He's also out of training. What am I to do?! >.<

Check your Battle Statistics page to see if the pet is already in a battle that hasn't been completed.

It's not, but after going there and back, it's working fine now. o.o weird