My son went through this and was another one of those toilet trained overnight kids I found when I noticed it was about time for him to do a poo I would put him on the toilet (bought the cusey tooshie) and say it was time to practise out pushing sounds and wait within 15 minuted he usually had results he got bored of that I would brush his teeth or something to keep him occupied becase just sitting on there long enough seemed to finally make him want to go