Certified GKH Gardening Expert
This can, often, be due to a lack of phosphorus in the soil, as well as too much nitrogen. If you do not believe this to be the case, then it could simply need a pH adjustment. Putting dolomitic lime in the soil will help buffer the pH close to 7, so that the plants are able to take up the potassium better.
There are a few other things that it could be and this article will explain things in more detail: https://www.gardeningknowhow.com/edible/vegetables/radish/radish-does-not-form-bulbs.htm