Last year I managed to grow some really good early caulis and cabbage, however I had the following problems with later brassicas. Cauliflowers produced good heads, then bolted in a few days. Broccoli formed tiny heads and then flowered. Brussels sprouts formed, but opened almost straight away.
Bolting, which is premature flowering and seeding of crops, is usually caused by plants being put under stress, in particular climatic stress. Temperature plays a vital role. In early spring plants prefer cold nights and warm days. If the days are cold too, bolting is likely. Mild autumns and winters will trigger this condition in certain crops. The factors are quite complex, and vary not only between crops, but even between different cultivars.
Cauliflowers dislike any check to growth, such as low temperatures, water or nutrient shortage, even windy sites!
Modern F1 hybrid cultivars of Brussels sprouts are far less likely to bolt, unlike the open-pollinated cultivars. It used to be thought that planting firmly was important to prevent sprouts from blowing. Our experience at Ryton is that this is not necessarily the case.