Try rubbing a clean white paper towel underneath the leaf and see if it comes away a yellowish or pinkish color. There are these little bugs that can get up under the leaf and you will begin to notice little yellow dots where they have pierced the leaf to suck out the nutrients inside. Over time it will kill the plant. Either get some insecticide soap or a good all around treatment. I found a mix at a big box store; it comes in refills along with the smaller aqua/blue bottle and works or 2-3 months at a time for me. It is a 3 way spray, it kills fungus, mites etc. If I treat on a regular schedule I no longer have such an overwhelming problem with the soft scale, mites, and rot. There is also a spray you can mix from water, alcohol and oil of peppermint. You use a couple tablespoons of rubbing alcohol to a spray bottle of water and a few drops of oil of peppermint. The last is for a pleasant scent really, not necessary, but it's lovely to smell and can be purchased at any grocery store in the baking aisle.