The Radius search functionality is built around the ability to search for polygons which allows us to retrieve the results that sit within those search polygons.
On occasion, a postcode will only represent a single building and therefore be a point rather than a polygon and as such will not necessarily contain data within it. To overcome this scenario, simply search for a wider postcode, for example, if you are looking for WC1V 6EU which is a point, please search for WC1V which will return a polygon and the data within it.
Or you can try drawing your own polygon. Please see article HERE on how to do this.