There could be a lot of reasons. (1)Different apples have different densities - I used honeycrisp. (2)Letting them cook a little longer helps them to soften enough to soak up more color. (3) Most of all, make them 2 days before serving and just let them sit in the fridge and soak up all the red syrup possible. If they do not turn as red as you'd like add a little red food color.