I had quite a few the same from the outside they looked full but were empty, I didn't know whether to blame saved seed from last years crop or lack of pollinators (they were planted out in March).
I can only say most of mine are OK. We're not exactly flushed with pollinators I must admit but we've had several feeds off our smallish row and dismantled it yesterday before saving all the knobbly pods to dry for 2023 seed.
@n.pennygar The problem of attracting suitable pollinators to your bean plants can be partially overcome by interplanting with other greenstuff, such as German Mustard (tasty salad leaf). The mustard generally grows rapidly & goes to seed after bearing masses of yellow flowers! The bees etc come along &, hey presto, call in on the bean plants too! Give it a try - has worked well for me for over 30 years [& still counting!].
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In the sticks near Peterborough