Ask Mr. Smarty Plants

Rudbeckia hirta var. pulcherrima (Blackeyed susan) appears on this USDA Plant Profile map to be native in or near Davidson Co., and you say your patch is established, so the soils should not be the problem. This plant is pollinated by bees and flies but also attracts butterflies, and is a larval host for the Silvery Checkerspot.

Do the petals look like they have been chewed at all, or are they stunted or misshapen? Take a look at this page of images of Blackeyed Susan from our Native Plant Image Gallery. There seems to be quite a variety of leaf types and colorations, but no closeups, at least, of flowers with stunted petals. If you find any caterpillars, they could be the culprit, and are probably the larvae of the butterfly mentioned above.

If you have eliminated all those problems from consideration, we can only fall back on the weather. Here in Texas, we are in such an extreme heat wave and drought that flowers bloom, set seed and go into near dormancy, just to survive. The Blackeyed Susan is a biennial or can be a short-lived perennial. This plant does not need to be fertilized, but it does like some moisture. Our best bet is that it is adapting itself to adverse conditions, but continuing to set seed (the black center), and hold on until the weather gets a little easier on plants and people.