brussel sprout seedlings - why no growth ?

I have quite a few seedlings, grown in 2 batches.  The first batch germinated OK, then got to about 4 lvs, before the outer leaves slowly died,  the plants are still alive, but not flourishing (been germinated for months now and are still  small.

I germinated a second batch and the same thing happened.  I transplanted into individual pots, they are in multipurpose compost, on S facing window cill, no draughts, cmompost damp.  What am I doing wrong ? Do they oject to being transplanted when small (I guess I treated them like tomato seedlings).

I feel that these are suffering from the effects of too much heat, brussels like cooler conditions otherwise they stop growing. If they're a good size I'd move them outside and into a cold frame, for a week or two.

thanks, I did wonder about sunscorch, so I had moved them off the cill a bit.  they are a bit tiddly still , but I might plant out and protect from birds and slugs.

I know what you mean about the compost - but they germinated well, it was only at about the 4 leaf stage they started to shrivel

thx