It could be the soil itself, if there is an over-abundance of nutrients then the vine has no need to reproduce; if the soil was harsher and the plant was a wee bit struggling, then there would be a need to reproduce and you would have flowers and hence beans. You could stop watering the plant for a bit, or cut down on the amount of water being given to the plant and see if that happens to do the trick. Or, in the alternate, you may have received a hybrid plant that produces nothing but leaves. =Birdy