What type of dog food are you using? If a dog food is basically filler, then its going to be harder. So try to get a dog food with more protein and definitely vegetables and fruits. I like
Dog Food Analysis - Reviews of kibble for reviews and gradings on things like that.
Canidae, Merrick and Timberwolf - are 5 star brands and you can get it at petco and places like that. I know at least we got Zig's Canidae All Life Stages there.