Phrases can be treated differently in databases. Some will automatically assume adjacent words are to be treated like a phrase but others don't. For those that don't, they require the use of (typically) double quotation marks around the specifc group of works to denote the phrase. Although some make ignore them, it is recommended you use the quotation marks when you enter a phrase just to be sure.
"weapons of mass destruction"
An exact phrase should find the works in exactly the order identified.