You need to get re-fit. Your shoes do not fit you correctly, pointe shoes should fit snug like gloves with your toes lying flat and the tip of your longest toe touching the end of the box when standing. When on full pointe you should only be able to pinch about 1/2 of an inch of material (satin) on your heel. When you have shoes that fit correctly, I recommend sewing a criss cross elastic with the elastic at the very back of your heel (to truly keep your shoe up and on) stretch to where your ribbon should be (right at the start of your arch). This will pull she shoe not only on the back of your heel but also pull the ribbon and shoe up into your arch giving a pretty and form fitting line. (three quartering the shank helps this too) You can also put rosin on your tights where the heel meets the shoe. They even make inserts that you stick on the back of your shoe that make the shoe grip to your tights. (Heel Grippers by Gaynor Minden) Good luck!!!