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

My battle pet Steven is bloated and very happy, he is fully healed and not in training...what else am I missing?

Have you checked the Statistics page in the Battle Coliseum? Your pet might already be in a battle that you haven't finished.

That was it exactly! Thank you very much for taking the time to help me out :D