There are several reasons. 1) Not using the proper ratio of ingredients -my recipe uses 1/2 cup of butter, 20 ounces of marshmallows, and 10 ounces of cereal. 2) Too much moisture evaporated, or the sugar was cooked too long and started to crystallize. 3) Too much grain was added, drying out the texture. 4) The cereal was pressed down when shaping -spread into an even layer, do not compress!