Babies not performing well

I have a bunch of babies that popped from seed nearly 3 weeks ago and they are performing badly. Most are still only a couple of inches tall, slightly yellow and the sprout leaves are yellow/shriveling up.
The seed stock is fine, so it's not genetics.
I'm using a 600W hps bulb about 5 feet above the plants. For the first 10 days I used a couple of 125W CFLs.
Temperature is between 25 and 27 degrees C.
I'm using a soil mix from the local garden center with NPK - 12/10/16.
With the later batch I mixed in about 40% coco fibre / perlite but performance is still very bad with all plants.
Today I flushed them with double the pot size of Ph 6.5 rain water. All the water I use is rain water.
This is the first time I've had this problem and it's really confusing me.
Normally I use BioBizz Light Mix but couldn't get hold of it this time around, could it be simply that there is too much Potassium (K) in the soil?
Any advice would be appreciated.

You can't flush soil that has food in it. The more you flush, the more you feed. The seedlings are too young to feed, and you have been feeding them from the start.

thanks for the insight. So I guess I'll just hope they get big enough to transplant before dying.

I would probably try to transplant them now. The nicer ones at least. It's a resilient plant, you would be surprised what they can live through. Actually, when they are young, I'm not real careful with them. I don't beat em up, but do handle not gently. When I water, I might knock the plant around a little bit, let the dirt dry out to where they start to wilt, maybe turn a fan directly on them for a bit. It might all be in my head, but I think it actually helps the younger plants a bit. Once in flower though, I treat them like the ladies they are.

OK thanks again, I got Green Label Total Mix but that's high in nutes as well. Do you mean transplant into BioBizz Lightmix or something like that (until they grow up a few more inches)?

Yea. Transplant into a medium that isn't enriched.