Is this happening to just your radishes, or is it happening to other root vegetables too? I have never grown radishes but I have had this issue with beets and carrots and found out the hard way that a lot of newer subdivisions will scoop off most of the top soil and only leave a little bit before you hit clay. The lack of soil prevents things like root vegetables from growing larger. If you know that's not an issue, then it's probably something else.