I have a Cherry Cola heuchera planted late June; it's not growing but it looks healthy. It was planted in black dirt with clay underneath. Another heuchera next to it is growing fine; same soil conditions.
New Heuchera Is Not Growing
Certified GKH Gardening Expert
Drainage in soil can be different from one planting location to the next. Knowing that you do have clay soil and that the plant has changed very little through the growing season, soil is very suspect.
You can do a soil test to determine what is needed, but drainage is a likely issue.
Here is a link to refresh you on the growing requirements.