Well are you using version 1 or 2? Anyway check the growth chart, then if you can get it, you most likely need to take good care of your gotchi. And if you have no luck at all you could always debugg your gotchi for him! ^.^
Actually, if you have a v1, on odd generations, keep all of its happy hearts filled up, but be very forgetful about its hungry hearts. That's how I got mine.