Seed starting

This week's gardening question is all about dying brassica seedlings.

Question

I have a brassica conundrum that I am hoping you can solve. 

I have, for years, struggled to grow brassica seedlings indoors. The seeds sprout just fine, get to about 2" high, then the leaves go pale, shrivel up and die. The stalk is fine, it is the leaves that die. 

Kale, broccoli, cauliflower and cabbage are all equally affected. This has been going on for at least 5 years. I've tried different soils, pots, lights, watering conditions, everything I can think of.