There are a no. of reasons for this, but chances are you do not have the proper mehndi / henna. Only certain areas of the henna plant can stain your skin. Most henna powders come from the whole plant being ground up. These powders contain spores, which take on a sponge like consistence when mixed. Mehndi / henna powders come from the top of the plant, the blossoms. This powder is very smooth and lays on the skin easily.