It sounds you could be having a problem with Bean Seed Fly which lays its eggs on organic matter in the soil. The maggots feed on the developing shoots as germination begins, thereby destroying your crop.
Make sure that you are using fresh compost - avoid using leftover compost from last year. Sow your beans in the greenhouse or indoors to avoid the flies coming into contact with them. If this is not possible then cover sowings and the young seedlings with a cloche or fleece to protect them.
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