The usual complaint is about flower drop, but in this case it sounds like the flowers have been pollinated and the bean is not able to grow for some reason.
I'd guess the most likely cause is lack of watering.
Runner beans like to be cool and damp at the roots, so try laying a seep hose along the base of the row close to the plants. Leave this in place to give them a good soaking where it matters, and you should see an improvement.

You could also feed with tomato food once or twice a week to see if this helps!