Seed failure

Where are you growing them, and in what container? 

The compost appears to have a green layer of moss on the top, which would indicate that the compost has been kept too wet (how much are you watering them, what's the drainage like and are they sitting in a tray / saucer of water?).  What compost are you using?  Also, are they in a heated environment or not?

I think they're a bit too damp. Also, I'm not sure that cabbages and other brassicas like heat.

Thanks both.  The seed batches that I sowed last year were over a period of time, so would have been from several bags of verve -and this year is a new one. Other plants from the same compost also seemed fine.  I bought new seed this year, so maybe it is the greenhouse heat that they don't like?.  maybe I should sow more seed in the cold frame and water less?. Thanks.

hi the compost could be sourced from municiple compost  its not made in this country so could be mixed with any rubbish there was a cautionary warning 2 or 3 years ago that amino coprydol lawn weed killer was still active after 9 months of composting,this faulty compost may have been shipped abroad  then sent back as a compost mix always check by smelling the compost if it smells chemically then it could be contaminated   cabbage need initial heat to germinate then cooler conditions for growing on i used sow 10,000 a day from March to July if any failed to grow the boss was not pleased   jezza

Leaf mould isn't usually advised for seeds because of the bacteria and fungi that will be in it.

I think your problem is as already identified - too damp and perhaps too warm on sunny days.

Leaf mould has a lovely structure which is suitable for sowing seeds.

However, seedlings once germinated are at risk of 'damping off', which is triggered by fungal infection.

https://www.rhs.org.uk/Advice/profile?pid=151

The advice from the RHS to avoid this disease is to use sterilised home-grown compost (or leaf mould) or commercially bought composts.