Answer:
There could be a number of reasons why your runner beans have not set. The most common reason is infrequent or insufficient watering. Runner beans are thirsty plants and it is essential that they receive balanced regular watering rather than 'drought and deluge'.

Containers can dry out particularly quickly and so should be given greater attention. A top mulch may help to keep the compost in pots moist for longer periods. Another cause could be cold wet or windy weather during the flower season. This will have prevented the pollinating insects from getting out and doing their work. Not a lot can be done about this now, but next season be sure that you position your beans in a sunny, sheltered spot.