I have two patches of Astilbe. One is under a Japanese Maple and doing well. The other is in a border right up against a cedar hedge and next to a hydrangea which is doing fine. This one is not growing well. Does it need more light?
Your astilbe is getting about four hours of light every day, and the hydrangea in the same vicinity is doing well, so light is not likely the problem. It could be that the astilbe is not doing well against the bigger plants in its competition for nutrients and water.
If you were to enlarge the border somewhat and bring the astilbe forward and away from the roots of the tree and shrub, you may find that it does better. Amending the soil with organic material and shredded leaves will also help it get a restart.