Dying cabbage seedlings

I planted some Primo and Grayhound cabbage seadlings about amonth ago in to 2" sq cell trays using multiper compost. Theseadlins were starting to develop their first trueleavesbut now havestarted to wither die.

They have been kept inside,germinated on a windows ill but have been in a room awayfrom direct light for the past week... and idears?...could they have been over watered?

There's no probably about it!!

Brassicas hate heat. I sowed my winter caulis in modules outside last month and they were up in 3 days & have just been potted on into 3" pots.

I sowed winter green cabbage seeds in a raised bed which I covered with enviromesh as it is sharing the space with some carrots.  However, on lifting the mesh today, I've noticed the seedlings are shrivelling up and not looking too healthy.  Does it get too hot under enviromesh??