Read wordpress search special characters for more information.
The problem with special characters (including apostrophes) that some of them have many different visual variations. For example apostrophe cahracters ‘, ’ and ‘ are 3 different ones. When making a search query, the database treats these characters separately. If the correct one is entered, it will match, otherwise it won’t.
Support for special character comes with advanced search features supported by third parties like ExpertRec.