The SharePoint Search is powerful yet very complex. There are a lot of settings and parameters that influence the search results.
For example; did you know that the Culture used in the query is used to determine wordbreaker, stemmer, and thesaures? Search with the wrong Culture and you will get unreliable and strange search results. I have seen it happen at a customer; the documents were all written in Dutch but the search was configured to use the English Culture (en-US: 1033). Finding out why some documents were included in the search result while others are left out can be a challenge as you have to check every search parameter at WebPart level, Site level, and Service Application level.