Within the soil are the nutrients upon which the plants feed. If the soil in which they are growing is overly rich in nitrogen, you may get some hefty radish greens, but the radish root suffers.
Soil pH is another factor. Radishes do best with a soil pH of 6.0 to 7.0, they will grow fine within a small percentage of these parameters but if the soil is too alkaline roots will sometimes not form properly. Too acidic leads to other issues.
Temperature is also sometimes a factor that can't be ruled out. In very hot weather radishes have a tendency to bolt to seed. The plant at this point will concentrate its energies on producing flowers and seed, and the root in turn suffers and dwindles. In addition to substandard roots so far as size is concerned they are often unpalatable as well.
Tips on Planting Radish
1.Choose a site with 6 hours of sunlight or more daily.
2. Loosen the soil to a depth of roughly 1.5 to 2 feet, which is the depth of the average garden spade.
3. Harvest Radishes planted in the spring before the hottest weather sets in, and you can plant another crop of radishes in late summer as the heat is subsiding for an early fall or late summer harvest.