Could be no pollination - you could get a paintbrush or a cotton bud and do the job yourself as there are not many bees around to do it for you.

It could also be dryness at the roots. Runner beans need lots of water. You may need to water them, trickle the water on slowly so that it's absorbed and gets down to the deep roots. If you just chuck a bucket of water over them, most of the water rolls off the soil when it's dry.

Try cutting the end off a plastic drink bottle and put it upside down in the soil next to the plants (lid off). Fill with water and it will trickle out into the soil and not run away.