The root of the problem
Little white grubs emerge and eat the roots and your vegetable show is over. Take action at planting time by using cabbage collars which you can buy online or from the garden centre.
If you want to save a penny or two you can make your own from pieces of cloth or carpet, cut into circles. Cut a line to the centre and then cut two snips to make a Y shape in the middle. Fit these tightly around the young plants at transplanting time and they prevent the female root fly from laying her eggs.