hi are you planting them the same plot every year.the crops need rotating.good luck

Perhaps its cabbage root fly. You can prevent that by fitting little circles of a suitable material about 3 inches diameter round the base of the stem (you need a slit in the circle and room in the middle for the stem) You can buy purpose made ones in garden centres. They prevent the fly getting access to the stem and laying its eggs there. It may be too late this year but worth a try, especially if you dust or spray round the base of the stem before you fit the collars.

Almost certainly its cabbage root fly , only sure way to avoid losses is to cover your brassicas with enviromesh netting , this family includes cabbage , caulis , broccoli , brussel sprouts , swede , turnip , radishes and wallflowers Chemical controls are undesirable and thin on the ground anyway .