Why Have My Brussel Sprouts Failed To Form Tight Buttons?

Brussel sprouts often fail to form tight buttons if they have been planted in loose soil or they have been overfed.  Sometimes the wrong type of fertiliser can cause the buttons to be loose.
Whilst it is too late to salvage them for this year, you can start to prepare for next year.  You should dig the ground now and ensure that it has plenty of time to settle down before spring.  Once you have dug over the soil, then add plenty of really well rotted farm manure.  Then dig it over again to ensure the manure is well spread through the soil!
Next,  tread the surface when it is dry, then rake away your footprints.  This will leave the soil quite close compacted, which will help.
Finally avoid any fertiliser which is particularly rich in nitrogen.  Ask your garden centre staff to recommend a fertiliser which is suitable for sprouts and next year the problem should be solved.