1. There is too much scale build-up in the steam heating reservoir. Descale your appliance regularly to prevent lime scale build up in the water tank. Please read the article on descaling.
2. There is not enough water in the steam heating compartment. Add the correct measure of water to the steam heating compartment (the water goes in the tank NOT the bowl).
3. There is too much food in the bowl and there is no room for the steam to circulate and fully cook the food. Reduce the amount of food in the bowl and/or run the steaming cycle again.