What varieties are you planting? And, what is the soil like? Carrots like deep, sandy, poorish soil, with low organic content, and excellent drainage. If the soil seems good, healthy foliage with little roots could mean harvesting too early, poor drainage, or a potassium deficiency.

Ah! Just read read your previous question, and "well manured" is probably the source of your problem. I would re-work one end of the bed, replacing some of the soil with coarse sand, tilling it all together, and growing carrots, butternut squash, and peas in succession.