Some pieces of food in the pot, steaming baskets or steaming tray are larger than others and/or require a longer steaming/cooking time. Set a longer steaming/cooking time. When you use both steaming baskets and the steaming tray, you can easily prepare ingredients with different steaming times.
You put too much food in the steaming baskets and on the steaming tray. Make sure you put the food in the steaming baskets and on the steaming tray in such a way that there is some space between the pieces and that some of the steaming slots stay clear to allow optimal steam flow. Please consult the recipe book for the correct quantities.
You did not put the lid on the appliance (or not properly on the appliance) when you were steaming. Always place the lid properly on the appliance when you are steaming, otherwise a lot of steam escapes and the food is not steamed properly.
The temperature is not hot enough. Always let the appliance heat to the right temperature before you steam the food.