When it gets really hot, mine cut down on food.
Also, I find that when they are fat and well fed, they have less interest in "ordinary"
food. They can go a long time on no food at all, or on way less than you think they need.
They are probably not hungry, and are bored with pellets. I bet that if you offered a nice juicy feeder goldfish from PetSmart, they would snatch it up.