Indian Paintbrush grows fine in my front pasture with the bluebonnets, prairie verbena, and native grasses. In a fertilized flower bed composed of potting soil, it probably wouldn't be very happy. The roots of paint brushes need to tap into roots of other plants in order steal their nutrients; hence a "weedy environment" is crucial. Guess you could call it a Vampire Plant!
My natural soil has a lot of limestone rocks which add to the alkalinity so that probably helps too.
You are in a good zone for them so you should have good luck as long as you don't pamper them. Hope this helps. Don't give up!