Not eating "full" is not a problem and can happen but if it is going off food completely for more than a day then a vet visit is in order.
In the wild, animals get to eat only once in every few days and not daily and regularly. However due to domestication, dogs have become accustomed to regular meals at regular times and can learn to want food exactly at particular times as they thrive on predictability. However, occasionally it appears that they do tend to fall back on their ancestral roots and may skip a meal or gorge themselves. If it happens more than once or every few days, since pups are prone to intestinal parasites and infections, a visit to a vet is advisable especially if the pup is looking dull.