We have 2 rows of broad beans in full flower and the bumble bees are busy visitIng the flowers all day, every day, yet I've seen no sign of any beans developing. Why would this be?
PATIENCE
If you have flowers you can't have beans.
The Beans follow the flowers.
May I suggest you remove the growing tip off all your plants to reduce the chances of getting blackfly.
Usually I carried out this exercise once five sets of flowers had formed.
I learned this the hard way i.e. any time I went for six or more sets of flowers I was invaded with Blackfly.
http://www.thegardenersalmanac.co.uk/Content/B/Beans-Broad/Broad%20Beans.htm (http://www.thegardenersalmanac.co.uk/Content/B/Beans-Broad/Broad%20Beans.htm)