Use parentheses to separate keywords when you are using more than one operator and three or more keywords. While full Boolean operators are usually accepted only in the advanced search option of search engines, implied Boolean operators are accepted in the basic search options of most search engines. 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. Some search engines, in order to cut down on response time, routinely ignore small and common words that appear throughout the texts of documents and in titles.