The most likely causes are too much shade or too much fertilizer. They like light shade, so they should get some sun, even if it is through tree branches. They need very little added fertilizer. Too much gives you nice green growth at the expense of flowers. Remember they are used to blooming without our "help". A small amount (1/4 of what the label recommends) in the spring and again at the end of June or after blooming should be enough. Don't fertilize after mid-July. They also need to be moist, but not wet, until at least mid-summer. Too much water will drown them.  

It sounds like you are doing something right to have the plant flourishing. Good luck with blooms next year.