Why aren't portfolio pages appearing in google indexed files?

I have more than one category within my product portfolio, https://www.kirleystove.com/gas-fireplace-products

The “gas inserts” and “vent-free” portfolio pages appear when I view site:kirleystove.com on google, but “gas fireplaces” do not appear. What could be the problem/fix to get these pages to appear?

The only thing I can think of is I may have changed the category name of gas fireplaces or the slug in Yoast at one time, and this could possibly be causing confusion.

I do get an error in google search console, which may be from deleting a category previously so this may not be relevant.
1 Warnings URLs not accessible When we tested a sample of the URLs from your Sitemap, we found that some URLs were not accessible to Googlebot due to an HTTP status error. All accessible URLs will still be submitted.
HTTP Error: 404 URL: /product_cat/gas-fireplace-inserts/