Brassica Issues

I'm really struggling with getting any of the brassica family to grow this year.

No chance of any photos as I can't make them work but I will describe.

The seeds get going - 2 first leaves - fine.
Once the first true leaf comes through, the seedlings start to wilt in on themselves from the baby leaves inwards.

Growing in the greenhouse didn't work - thought it got too hot for them.
Same again in the garden - maybe too wet or too cold?
Brought them into the utility - it's darker in there but a fairly stable temperature as I don't use the radiator in there and managed to drag a couple of seedlings through.

Not sure what I'm doing wrong. Have normally been ok with Kales especially but this year have had to buy more young plants than grown any from seed.

I've tried normal potting compost, seed compost.
I've tried watering from above and from below, minimal watering, almost drowning!

Any ideas would be gratefully received as I do love my greens!