Every opponent is the Battle Coliseum says " Oh no! None of your pets are eligible for battle. You might want to make sure that they're well fed and happy! "

I really only wanted to hit the newest opponent so I can get at least noticed as a participant. Meijah, my pet I usually use for any battles although he's nothing remotely special, has full health and happieness as well as full hp and is unable to challenge any opponents.

you may be stuck in another battle. dunno which link it is, but try every link at the battle colosseum page :)

the link you want in the top of the Coloseum is Statistics. It will tell you if you're stuck.