This is a good question because it happens to many people. Your plants are most likely going to seed because of lack of nutrients, the pH of the soil is not right for them, drainage is not good enough, or a combination of the three.
Brassicas are gross feeders, so your soil needs to be full of nutrients. Add animal manure (sheep pellets or chicken poo), Natures Organic Fertiliser (full of minerals) and compost (which helps with the soil structure and retaining moisture). Brassicas need to be planted in good soil, fed at the time of planting and again during the growing period. Regarding the pH of the soil, all brassicas like to have lime applied to the soil before planting. And lastly make sure the soil you’re planting them is free draining. Having a slightly raised bed improves drainage. Put simply, plants go to seed when they’re stressed. Click here for how to create great soil.