SEO URLs only work for 2 out of 4 categories

Hello,
I have an .htaccess file that appears to be in good shape (mod_rewrite is ON). I have SEO keywords assigned to my parts. Use SEO keywords is turned on in Server>settings.

I am able to browse to successfully and see SEO URLs for products in 2 of my 4 categories. When I browse to products in my other two categories, I get a 404 error.
If I simply hover over the product name or image, I can see the SEO URL using the category name, but clicking on it results in 404 error.
If I SEARCH on the same product name and hover on the name in the search result set, the URL does not contain the category name, but I can click on it I successfully go to the product page. I am certain that my products are assigned to categories...

I am so perplexed! I am using a Template Monster template. I have contacted them and their solution was to turn off Use SEO URLs: NOT the solution I can live with! They recommended I check my .htaccess, which I have done. I host on Site Ground and Site Ground says as long as I put RewriteEngine on in .htaccess, then it is on. Plus, SEO URLS work for 2 out of 4 categories.

As it so happens, both categories where the SEO URLs works have sub-categories. Where SEO URLs do not work, there are no subcategories: just a list of products are displayed.
I have looked at the database tables for these products and categories and cannot see anything unusual.

Thanks very much in advance.

Not sure why, but when I renamed the categories by removing an 's' at the end, they both worked. This topic is closed.