Overcrowding or shade can limit beet root development, as can overfertilization. Have a soil test done to determine soil fertility and pH level. Contact your local Extension office for a soil test.
Beets are sensitive to boron deficiency, which causes blackspot, sickly growth, and poor taste. Only minute amounts of boron are needed, and if you use compost, the soil will be adequate. If you're in doubt, sprinkle a little borax around the plant.