Same problem here. DD1 especially would just hold it for days and ended up with severe constipation. We did a treat based thing after everything else had failed - they got a butterfly sticker for each poo they did, and once they had three butterfly stickers we went to the toy shop and they could choose any toy they wanted. Thankfully they never wanted much!! It took a couple of weeks but after they managed the first poo in the toilet there were then no problems.