They may have been struck by 'damping off'. This is caused by a fungal disease that attacks young seedlings. You can water with a fungicide to prevent it. Brassica seeds germinate quickly, so do some more, they will soon catch up.

Thanks. So is there nothing I may be doing wrong?

Try to use clean trays and pots ( I don't often clean mine either!!) and make sure there is plenty of air circulation. Pick out any patches that do go mouldy to help stop it spreading. And hope for sunny weather...