Sweetcorn I normally get 95%. I sow two packets, 70 seeds, at the beginning of May to plant out beginning of June. They need some warmth to germinate. They are big enough to plant out after the last frost.
I think you are sowing too soon. I sow courgettes, runner beans and french beans at the same time. All of them I sow singly in 3 inch pots.
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