It sounds to me like some type of pest. Examine the leaves very closely, some pests, particularly caterpillars are very small and difficult to detect. If you do find caterpillars, you can use thurcide on the plant. As always, mix according to the directions and spray both sides of the leaves and around the plant as well. It is best to do this in the early evening because exposure to early day light may cause spray burn. Thuricide only works against leaf eating caterpillars. It is harmless to most everything else unless there is a specific allergy to the ingredients.