Broccoli (Sprouting) Bordeaux seedlings - please help!

Hi there,

I sowed some Broccoli (Sprouting) Bordeaux seeds at the end of February in small 7cm pots and they were doing fine until about a week ago. The majority of the seedlings seem to have died / shrivelled up now though. I water the seedlings every couple of days and they are situated on a S facing window sill at home. The tomato and pepper seedings that I sowed at the same time and are on the same window sill seem to be thriving. Does anybody know what I may be doing wrong with the broccoli seedlings and why they are dying?

Many thanks
Lee