1. High temperatures: This is the most common cause in Kansas because beans prefer temperatures between 70 and 80 degrees F. If the temperature remains above 85 degrees, the flowers often drop (blossom drop) or they dry up on the plant (bud blast). Hot, dry winds can make this condition worse.
2. Extreme fluctuations in soil moisture: Too much soil moisture can be as harmful as too little as excess water limits oxygen reaching the roots. Extended rainy periods or extended drought can cause bean plants to produce few pods. We can irrigate during dry periods to alleviate drought but it is difficult to deal with excess moisture other than to use raised beds or berms to grow beans.
3. Letting pods mature on the plants: Maturing pods cause the plant to put energy into making seed rather than forming new beans. Keep plants picked even if pods are too mature to use. (Ward Upham)