interesting post here by him:
"As a rule your natural hunger is the best guide, as I don't believe in force feeding. Sounds like you eat pretty clean, but if you're not gaining as fast as you believe you should, then look at two variables: 1-protein, eat an extra 24-50 grams of protein and 2-keep tabs on your training volume and intensity, for example some people overtrain to the point where they lose their appetite.
Good luck,"
overtraining causing a lack of appetite, interesting.