If your leaves are healthy, and there's a lot of them but they're just not 'hearting', then the problem is most likely that they're not firm enough in the ground. This can also cause a prblem with brussels sprouts - the sprouts aren't tight little buttlons, but more like loose flowers
The answer is to walk over the soil before you plant them. Yes, that does seem contradictory when you spend ages making nice fluffy soil, but it's what they want! And then when you plant them, actually tread the plants in with your heels. I couldn't bring myself to do it the first time I planted cabbages & they ended up more like spring greens
