Woocommerce theme conflict

We have a WP theme by a third party and recently installed Woocommerce. All works fine until we changed the Shop Page Display in the Woocommerce -> Settings -> Products -> Display to show categories and subcategories. The categories show up fine in the Shop with the correct numbers of products in each category, but when we click on a category, no products show with the message ‘No products were found matching your selection’.

If I change over to the Twenty Fourteen theme, all shows correctly. What is the best way to find and fix the issue? I am a PHP developer and used to delving into the source/core.