Ask Mr. Smarty Plants

It's difficult to tell you what, if anything, you have been doing wrong, because we don't what you have been doing to transplant Rhus lanceolata (prairie sumac) from its natural setting to an urban property. However, we did find this propagation information on the webpage linked above on Flame-leaf Sumac:

"Several species of Rhus are commercially available as propagated stock, which is both ecologically more desirable and usually more successfully established than plants dug from the wild. The best time for planting most shrubs and trees is during the dormant period of fall and winter. Even during the winter, however, the root ball needs moisture, so plan some winter watering if soaking rains fail to come regularly. You can use a spade to cut outer shoots from a spreading cluster in the wild. This allows the plant to remain in its natural setting while providing transplantable shoots that already have a developed root system. Simply cut straight down between the outside “sucker” shoots and the other plant and then cut around the new shoots to remove them from the soil. Keep the roots wrapped in damp newspaper and out of the sun. Replant the same day, if possible.

Even during the winter, the root ball needs moisture, so plan some winter watering if soaking rains fail to come regularly. As with any shrub or tree, native or introduced, the first year requires regular deep watering for successful root establishment. Once established, sumacs do not require fertilizing or watering beyond average rainfall."

The sumac does put up suckers that will eventually add to the plant and form a thicket. If you have been attempting to transplant one of these suckers without finding one that has roots already developed on it, that may be the problem you are encountering. Another problem may have been transplant shock. Following the instructions to transplant when the plant is semi-dormant, late Fall or early Winter, getting it quickly back into dirt, and watering slowly and consistently can help prevent transplant shock. 

Rhus lanceolata

Rhus lanceolata

Rhus lanceolata

Rhus lanceolata