Possibly the ground was still a little cold when you sowed the seed. I wouldn't worry they will come through.

As Mg says its too cold. My carrot seeds are only just through the soil as is the onions and beetroot. My peas are still in the coldframe. Cover them with fleece

I too planted some carrot seeds 3 weeks ago and they are only just beginning to show through. I'm in a fairly southern location (about level with Oxford) so if you are much further north then it could be another week or 2 before you see any signs of growth. After such a cold winter it will take the ground time to warm up so eveything is likely to be slower germinating this year.

We planted carrot seed in the control bed today (read my blogs on growing by the moon) and will sow in the moon bed on Sunday (Perth Show on Saturday). We covered immediately with fleece - not so much for warmth but to keep the dreaded root fly away. We also planted the carrots between two rows of onions to confuses the little so and so's!