Achilleas don't usually suffer from pests, did you see any before you sprayed? They are quite late to flower, mine are just starting to produce buds now.

Two reasons that would cause this is too much fertilizer & not enough sun.

Yarrow thrives in full sun and very well-drained soil. They like hot, dry conditions and low soil fertility, but won't tolerate wet soils which promote powdery mildew. It's a little early to expect blooms. As Landgirl mentioned, they are late bloomers.