Answer: Astilbes do need a rich, evenly moist soil to grow and bloom their best, so it would seem you have taken care of the basic requirements. They also prefer a partial shade location and can "burn" in too much sun; excess fertilizer could also cause "burning" so make sure you are not exceeding the dilution amounts on the label. Also, the dry season we had last summer and fall weakened many plants so they were not able to overwinter successfully, so this could be a possibility.
Voles would be a potential cause of problems -- they love hostas-- although with close inspection you would be able to tell that an animal had literally eaten away the roots.
Based on your description,it is difficult to make a definite diagnosis. There are several pests and diseases that could cause wilting of astilbe, including cercospora leaf spot, powdery mildew, fusarium wilt and the underground feeding by grubs of the black vine weevil. You might want to consult with your county extension to see if they can help you identify the cause of the wilting and suggest what steps to take next.
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