I try to grow Wachren winter caulies.( A very unfashionable seed these days.)In well manured spots. I think the secret to very large heads is to sprinkle sulphate of ammonia around the stalk as the head starts to develop ,about 35grms,water it in well, but dont let it touch the plant in case of burns to it. It is safe to use and guarantees a large size head that you require. Could it be a PH problem?