I've had an allotment for 7 years and although have been able to grow most things successfully I've never succeeded in growing cabbages and brussels sprouts well.
The soil is not bad; closer to clay than anything else and every year re-cycled compost (not manure) is added to the beds.
Seed is sown in a seed bed and I make sure the transplanted plants are bedded in tightly and feed at various times, usually with Growmore and Miracle Grow. With sprouts I always get good plant growth but no sprouts or else the sprouts burst early. Cabbages seldom bulk up.
Any help appreciated.