Do they have much sunlight?
I also try every year without much success but this year I decided to grow them in pots.
I had two pots, both situated in different areas. One in an area that gets both sun and shade (I'll be lucky to get 6 carrots!!), and the other in a sun trap area (the pot is bursting with carrots, so much so that I'm going to have to this them out).
I'm no expert when it comes to carrots, but worth a try??