He's been potty trained for about 3 months now and always does wee-wees in the toilet,but for the last 2-3 weeks he has done a poo in his pants almost every day without fail. He knows what he's doing,as afterwards he'll tell me he's done it, and when I ask him where he should do poos he always says in the toilet. I've tried incentives,but he just doesn't seem to be able to get the hang of it. Why? And what can I do to move him on from this?