The usual reason is loose soil. they need planting with 'an ammer' as my old guru used to say. We plant ours deeply, to the bottom leaves of the young plants and tread them in well. They do not like to follow any root crop where the soil has been disturbed in harvesting for that reason.
The 'blown' sprouts are still very edible, just treat them like small cabbages. As was pointed out to me the other day, at least with blown sprouts you can see that there are no little black aphids inside!

I wasn't the one to ask the question but I am so pleased to see your reply as I am never sure how deep to set the plants I start in house. We grow our own veggies to last from season to season so it is gardening on a large scale for us.
My dog loves the loose sprouts, also the cut off ends steamed served with his doggie food. The vet told me to always supplement dry dog food. My last dog loved them raw.
Clementine

I found Bubbles the best brussel to grown for my land, sandy soil. Only one place I can ever find them and buy via catalogue. I am in Canada. I tried Bedford but had no great success.
Clementine

Many thanks for your replies,
All are useful and noted, I think I`ll revert to Hybrid seeds next year,
Apologies for the glaring spelling mistakes,I was in half light when writing.

Do you know I did not even notice the errors of typing!
We grow a 'sweet' variety as the old fashioned ones. like Fillbasket etc. taste incredibly bitter to me because of the tablets I have to take. The only ones we fail with are the Red sprouts, they do not seem to grow as well as the green ones.

They will of course still be edible and just treat like small cabbages. Not what you wanted but salvage something from it.