Why does my clematis have green flowers?

Camelliad

What is happening to my clematis? It's called Jeanne's Pink - I planted almost two years ago and it flowers twice a year. In spring it has (should have) pale pink double flowers and in late summer it flowers again with single pale pink flowers. In spring last year it only had a few flowers, which came out green, but then the second flush in summer were single and pale pink so I thought it was just "settling in". 

This year it has put on lots of growth, lots of flowers but again they're green. Has this happened to anyone? It seems otherwise healthy so I was hoping it was a passing phase...