And what has the weather been like where you are?

Well, hot, dry weather will cause them to bloom prematurely, and lack of nitrogen will cause the same. If by fed, you mean lots of commercial compost worked in, that normally doesn't often have enough nitrogen for Brussels sprouts, which are medium-heavy feeders. Snoopdog, is there still time for a fall crop from seeds or small seedlings in the UK? That would allow Blackcat to try again, after working in some blood meal or fish meal.