The Boolean NOT or AND NOT limits your search by returning only your first keyword but not the second, even if the first word appears in that document, too. Use parentheses to separate keywords when you are using more than one operator and three or more keywords. Boolean logic is not always simple or easy. Different search engines handle Boolean operators differently. Some require the operators to be typed in capital letters while others do not.
Some search engines use drop-down menu options to spell out the Boolean logic in short phrases.
This could radically alter your search. Be sure you know the defaults of the search engine you are using and how your search engine handles Boolean operators or your search results may not be what you expected. Strange things can happen for other reasons as well. Whatever the case, you may never know the real reason why your search retrieves so many irrelevant responses. Not all search engines recognize the same stop words.