With ours, the doc said 'they'll grow out of it'. So I took them to a naturopath who ordered a blood test, can't remember what it's called but it checks for intolerance to the main culprits, dairy, nuts, gluten etc. Results showed nuts were the issue in varying degrees, eliminated nuts, took a probiotic, poop firmed up, and reintroduced nuts about a year later. They seem okay now, not that I really looked at their poo once they toilet trained.