Why don't my seedlings make true leaves?

I'm trying to start seedlings inside in order to lengthen my growing season (I live in northern New England). I have heating mats, and grow lights. I'm currently trying to start tomatoes, brussel sprouts, and onion. I planted them 6 weeks ago. I had no trouble getting them to sprout, and make seed leaves. They aren't leggy at all. But the growth seems to have stalled or stopped altogether. Can anyone advise why they aren't making true leaves?