Are these self fertilising if so perhaps thats the answer as I have read white flowering ones self fertilise. Other than that they like to be deep in soil fed and well watered they also like warm weather but will grow in colder weather.

Did your beans dry out earlier in the season?, this can result in flowers not setting. Also did you mulch the soil in early summer to maintain moisture and spray the flowers with a fine mist to help the flowers set?
Most importantly did you prepare the soil last winter?

Overfeeding can result in extreme foliage growth as well.

Have you planted them in a good sunny position?

Some years you will get loads of beans and in others few if any.

I planted 'Scarlet Emperor' they have done really well and are still covered in flowers, despite being disturbed during some pipe laying work.