Many types of beans are actually very poor as far as legumes go in fixing n. Bush beans are very poor, relative to peas. Faba beans are excellent.
We never innoculate our bush beans at all, because they only fix such a small portion of their n needs, so we fertilize in a side dress fashion if necessary. We put our legumes on the prior years corn area, so the corn has used up a fair amount of nitrogen which was provided with composted sheep manure.
We have been growing a purple bean variety for several years, and have really liked it. It grows small plants, but pods profusely and for a long time, relative to any greens we have grown so far.
Of course each year is different as well. Some years for reasons unknown, some crops just do not do well. I hope this is your issue.
Good luck. We love our beans!!!