Root crops need a deep, loose, well-drained soil to grow well. Is the soil in your garden compacted, or heavy and blocky? If so, work in some organic matter. As organic matter decays, it releases compounds that make soil more "workable," as well as promoting good moisture retention and providing nutrients to plants. You may find that building a raised bed makes a difference for your root crops, since they'll have a looser soil to grow in. As long as you thin the seedlings and water your crop regularly throughout the growing season. Another thing that comes to mind is fertilization. If you feed root crops too much nitrogen, they'll produce a lot of top growth and little in the way of roots. Have your soil tested to see if you need to balance your soil's nutrients -- that may make the difference.