Answer: Unfortunately based on the information in your description I am not certain what is happening. This is a very low maintenance and disease resistant plant, so there is no "usual" problem to suggest. It is possibly related to over or underfertilizing or some other soil issue, it might be the start of a disease such as black spot (very unusual on this plant) or even something in your water such as salts from a water softening system. I would suggest you work with your county extension and/or your professionally trained nurseryman to see if they can diagnose the problem for you. They may appreciate photos of the overall plant and closeup, as well as possibly some foliage samples enclosed in a clear plastic bag and kept cool so they don't wilt too badly. I hope it is nothing too serious.
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