Hi there. Thanks for asking this question. It is hard to tell for sure what might be causing the low yields, but here are some of my thoughts.
1) Drought stress can cause blooms to drop - beans need at least 1 inch of rain per week.
2) Intense heat can cause blooms to drop. Mid range temperatures with high winds can equal heat stress for plants.
3) Not enough direct sunlight can cause poor fruit set.
4) Too much nitrogen can cause lots of dark green leafy foliage but no fruit, adding phosphorus and potassium fertilizer will increase flowing and fruit set. Beans are a legume so they will fix some of their own nitrogen.
For more information, check out one of these fact sheets:
Please ask any additional questions!