Brassicas

Firstly...thanks for all the help from my previous messages...on spring cabbage failure and recently, how to manure the plot.No! I haven't lost it yet.
once again I'm going to attempt another 'batch' of spring cabbages and hope they 'heart-up' this year but the latest problem is my brussell sprouts which seem to have 'blown'.I'm told this is the expression.What is the reason for this and how do I prevent it happening again? Somehow the cabbage family and I don't appear to get on very well although my chard & spinach did well.