Ask Mr. Smarty Plants

Nothing that we have looked at so far on this plant, including that it is native to Grimes County, gives us a good answer to your question. Malvaviscus arboreus var. drummondii (Turk's cap or turkscap) does well in part shade (2 to 6 hours of sun a day) and shade (2 hours or less of sun). So, we'll dig a little deeper (pun intended).

You can follow the link above to our webpage on this plant to look at the Propagation portion, from which we extracted this information:

"Large clumps of Turk’s cap may easily be separated in early spring and transplanted to a new site. Be sure to water well."

However, this does sound to us like transplant shock, which can show up for years after the initial planting. Since this plant spreads naturally by layering, you may not have obtained a sufficient amount of root material with your original transplant. We consider Turkscap to be a semi-woody plant, which means it should be transplanted in cool weather, preferably late Fall or early Spring. Note the comment that it should be watered well after it is planted. There is, indeed, always the possibility that the oaks under which you are trying to grow this plant are fighting off this competition with allelopathy. You say your cultivars are in the same area; how close together are the healthy cultivars and the ailing natives? If they are under the same oak trees, getting the same amount of shade, in the same soil, about the only conclusion we can come to is that the natives were damaged in some way in the transplanting. If you do it right away, while it is still relatively cool, you could try transplanting them again to a more favorable site and trim them back to about 6" to take some of the load off the roots. Leave some leaves, though, as those provide the nutrition for the whole plant.

And ixnay on the fertilizer. Native plants in their proper environment don't need fertilizer, and the fertilizer can push the plant into more growth, putting more stress on an already-stressed plant.