Meconopsis Lingholm seedlings dying

I have four meconopsis lingholm seedlings which have germinated and been repotted, and two are now dying.  When they germinated I was absolutely delighted and had read as soon as they developed their true leaves (no matter how small) they need to be repotted.  So I did: I put it in small pots with ericacious compost mixed with vermiculite and then covered them with gravel.  I then have kept them on my kitchen window sill (north-facing) and watered when they are dry.  But unfortunately, the leaves of two plants are going grey and the new growth has now shrivelled up.  Should I be keeping them outside?  Am I overwatering?  I know that they tend to like the shade, but are they not getting enough sun?  I don't know what to do - I would be most grateful for any advice as I don't want to lose the rest of them and if at all possible, save the two that seem to be dying.

I don't grow meconopsis, but do know that they are not easy.

You say that you water your plants 'when dry'. I wonder if they are actually drying out. Just because a plant likes to be well-drained, and meconopsis do, does not mean that they want to be dry. Although it seems paradoxical, there are many plants that like plenty of water, and good drainage.

Thanks very much for you reply Gary!  I'll water a bit more regularly - my other worry is watering too much as well, as initially I thought the greying leaves looked like mould (similar to how my more mature meconopsis looks in the garden when there has been too much rain or moisture).  

I think I discovered the problem: unfortunately, the seedings were damping off.  I read that the meconopsis seedlings should be watered from below to prevent this.  Alas, only one left among the 6 which germinated!  Wish I had known earlier.  Anyways, thought I would share this in case anyone is / was thinking of growing meconopsis lingholm (or any other Himilayan blue poppy).