Yes, ash from fires is good for potash / potassium but high levels of potassium can cause magnesium deficiency in plants. The clay soil will be holding it rather than letting it get washed away with the rain.
If it is magnesium deficiency then you treat it with epsom salts mixed with water as a foliar spray every two weeks. I'm pretty sure you can buy epsom salts from the garden centre now and the dilution will be on the pack.
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