Dear Marianne, Adding leaves to the garden soil should not change the soil pH enough to impact growing your beans or other vegetables. Beans had a difficult year with the weather we experienced. When we have extremes all plants behave differently. Vegetables may produce flowers but no fruit or not produce flowers at all until the plant can support producing the fruits or seeds. Snap beans are considered a tender vegetable and are typically planted in early May, so they went in a little early, yet if they came up and grew normally, that was the correct time for your garden. I would blame this one on the weather and try again next year. Consider planting some beans as you did this year and another row after May 10 and observe the difference.
Sincerely, Richard