Custom post type not showing on main category archives

I am part way through setting up a new content type called "Walks". I am preparing to add various new custom taxonomies and custom fields, however I have just noticed that the initial category i have been assigning posts to is not aggregating the results.

To reproduce:

Got to
Click on "Countryside Walks" under the page title.
(This takes you to which display no results despite there being six walks listed in this category.

Can you please suggest the cause.

Many thanks