[Sugar's Mom]

Does anyone know when mama stops stimulating babies to poop? My pups are four weeks old and two of them seem to be constipated. I have been trying to start feeding them for three days. i start weaning with esbilac puppy supplement, a drop or two karo syrup and a little gerbers baby rice cereal mixed to a watery paste. i am wondering if this is constipating them and I haven't seen mama stimulate them but I have seen them trying to poop with no luck. I fed the same last litter and had no problems. Don't know what's going on here.