If the leaves were hanging down, I would guess insufficient watering. But are you saying the entire plant flopped over? Because bush beans tend to do that unless they are growing in absolute full sun... and even then, they flop over once the weight of the bean pods start weighing them down.
11 inches... how tall a seedling is is not always an indication of good growth since it could be due to insufficient light and spindly growth. Also, how many seeds did you sow per 1 gallon pot? I think there should be no more than 2 max. 1 each would be better. ...oh wait, you said you transplanted --- I think it's better to SOW beans where they are to grow. Although I do sometimes pre-germinate beans to ensure growth from every bean seed, I sow the just germinated seeds as soon as root tip appears.
Also, I would use s balanced fertilizer since beans are fruiting plants. Did you use rhizobium inoculant for beans?