Because you say that even the small Astilbes have had poor bloom, I suspect that it is because of the cow manure, which is commonly heavy in nitrogen. Fertilizing with too much nitrogen will cause lush leafy growth at the expense of flowers. There is probably nothing to do this year about the bloom. To ‘use-up’ the excess nitrogen in the soil you can mulch with something like wood chips or straw; these mulches need a lot of nitrogen in order to break down. You might also want to add a little bone meal to encourage root and flower growth.
In regards to dividing: Astilbe roots can become congested and old which will cause a decrease in bloom. This might happen every 4-5 years depending on the variety and growing conditions.