cupcake help

Could someone tell me why the cupcakes I am making do not puff up? They just stay flat across the top and run over the sides. I have tried putting less batter/more batter. Could it be the oven temp, or ingredients? I have been cooking for a long time but am rather new at cupcakes due to a Granddaughter who loves to bake. Thanks

Check the expiration date on your baking powder.