Whenever I use the search function in the Help Centre or even the Insert Article as Link in the article editing window, the search does not always find valid articles even when I know for certain that those articles exist. 

Even when I put in a keyword which I know for certain exists in the TITLE of an article, the article does not show up in the search results.

Is there something I am missing here? Why doesn't a search find all relevant articles?

(NB I have set the search to look into the whole of the Help Centre rather than the section, etc).