yeah, I don't think there is a cheat. But it depends on what kind of pet you have, like I had one and I only like had 1 bar even when it was an adult, and then my another one I had full when it was a teenager. You just have to watch them and make sure you know if they need to be praised or they need to have a time out. hope this helps!
Teens seem to require the most training; I always get the training bar full when it's in that stage. Punish it when it beeps for no reason; praise it when it has its back to you or when it's crying.