Common causes of excess or uneaten cecotropes are a diet too rich in carbs or protein, dental issues, medications, arthritis or overweight and the rabbit has difficulty reaching down there, young rabbits with short attention spans, rabbit in a new area and or some sort of change causing a disruption to the rabbits life.
For your rabbit, if he is eating his hay and grass well I wouldn't think dental issues as likely. It may be possible he is getting too many carbs from treats and pellets. You can try cutting out fruit and carrot and see if that helps, but I would suspect that you will also need to reduce the pellet amount some. Another possibility may be if the grass is young growth and too high in protein, but this wouldn't seem as likely a cause as too many carbs.