[Aunt Sally]

DD uses a large lawn roller on his brassica plot 

Staking the brussels to prevent rock would be a good idea.

[DD.]

I do indeed, but always stress NOT when the ground is soggy!

Like today. 

[brokenglass]

I know the wisdom in growing any Brassica is to make sure the ground is very firm but I grow all my Brassicas in raised beds where the soil is anything but firm!       I do get good results, the PSB is still producing a bag full every 3 days from plants which are planted about 1 ft apart, again this is against most spacing advice.

Not saying that what I do is right just that I do get good crops without the firming and wide spacing

[Aunt Sally]

I love PSB and have just stopped cropping my 2 plants (oodles for two people).

Perhaps growing them close together has helped them support each other.  It's wind rock that really damages the roots.

[seaside]

Brussels are difficult enough at the best of times... they need planning and preparation, and/or a huge slice of luck.
On our allotment site, last year saw roughly 60% of sprouts grown at the correct time fairly useless... they make good compost though. I wouldn't have thought sowing this time of year would help the odds at all. Mine are nearly a foot tall, well settled in and I am only 60% hopeful.
Buy some plants in, if you can find a good confirmed F1, if that is at all possible at a garden centre at this time of year, and even then they might be a bit late what with transplanting etc. etc.
As a novice, I was confused when Brussels were picked in February. I thought that meant they kept on growing apace through the Winter. They don't.