We have several UK based Dynamics 365 instances and were looking at publishing a over 1,000 KB Articles to one of our Dynamics 365 environments. After we had finished the usual process of creating the articles, adding to categories, reviewing them, and finally approving we were expecting to see these articles in our Cases form when we searched for our KB Articles, and we didn't. 

After a lot of head scratching and googling we eventually stumbled upon the solution, which I thought I should share as I didn't find anything else on Google that helped with our specific problem.

Being a UK based company with Dynamics 365 instances in the UK we naturally set the KB Article language to "English - UK". We found that when a KB Article language was set to "English - US" we could see the article in the search results. After a bit of Excel online data manipulation we changed the language of all our KB Articles to "English - US" and hey presto, we could finally see all of our articles in the search results for our cases.

TL;DR - If you're in the UK and you set your KB Articles default language to English - UK then the KB Articles won't appear in KB Article search results, even if you've done everything else correctly. Change the article language to English - US to fix this.