There are a couple of things to check when trying to battle.

1. Have you adopted the pet in the past 30 days? If so, you have to wait 30 days after adopting to use the pet for battle.
2. Other than happiness and hunger, you have to make sure that the pet has hp. If not then take it to the healer.
3. Your pet may be stuck in an old battle. Check the statistics page to see!

As Evy said, have you checked to make sure that they are not already in the BC?

Is this still an issue for you?

Glad to hear! Locking this up as resolved! :)