brassicas and salad leaves going white

This is happening again. I sow curly and Italian kales, cabbage, salad, mustard greens and after a good, healthy start the leaves at the tips go very white, shrivel and then the plant withers and dies. I am not over watering or anything and using new compost.

What could this be ? 

they gradually get lighter over a month or so until they are almost white and then die. 
sown in new seed compost

Seed compost is low nutrient so anything left in it for a month or so without being potted on or fed is likely to starve to death.  Brassicas are also very tough and don't need a lot of heat - good light but not direct sun or cossetting too much  

I'd agree that seedlings with just seed leaves should not need feeding.  I would normally only feed after they had got a couple of sets of true leaves. I read your post as true leaves going pale and then the plant fading away, didn't realise they never got going in the first place - sorry    

Have you lifted one of the seedlings that have been stuck at seed leaf stage for a month to see if they have decent roots?  Just thinking it could be compost too wet and roots rotting away    Maybe some more info about exactly how you grow could help, for example are you growing indoors/under glass, is it just normal plastic pots/peat pots and which compost you are using  

Tall seedlings could be lack of light or too warm.  I'm propagating my hardy stuff (salads, brassicas, chard, leeks ect) in a cold greenhouse and cold frame.  No extra heat and the doors left open to help ventilate during the day.  

It's been overcast here most days but stuff is developing fine    Have you got somewhere outside but sheltered to have another go KG?  

About 3 years ago a section of my lawn went completely white.I made enquiries and sorted it with a nitrogen feed.  Prior to this I had been making a toybox and had been sanding down a lot of wood. The wood dust was causing a nitrogen shortage and turning the leaves white.
All compost manufacturers are reducing the peat content, and I too am noticing a poorer germination. They do not say what is in the bag. Mine has 'carefully formulated' and 'added nutirents' But what if they are adding sawdust or other woody items. Once the nutrients are used up there could be a nitrogen deficiency, hence the white leaves??