Its sounds like either lack of water, or possibly birds, some are partial to Runner bean flowers. I would try watering and put some sort of bird deterrent (not netting you will find it difficult to get at the beans when they grow). You could try old CD discs on sticks, tie them loosely so that the move when there is a breeze, the reflection tends to frighten the birds.
You could also try putting other fairly tall sticks (old off cuts from trees with lots of thorny projections in amongst the beans to make it difficult for the birds to land (if their wings touch the sticks they won't try to land.l
Good luck

Spray with a teaspoon of Dettol in a pint of clean water
late evening. The birds dont like the smell, and leave the black fly alone, so dont knock the flowers off. The insects like the smell and pollinate the flowers.
You get Runner Beans 3 weeks early.