Set aside bias and expectations about what you think you will find and approach the research process with an open mind. Search two or three (or more) databases to comprehensively explore the literature, and follow a systematic search process as described below. Remember, "evidence" is a group of facts. One study is a piece of evidence - several studies resulting in complimentary findings collectively provide evidence.
This happens for many reasons!
Don't abandon the work you've done! If you're getting result lists but they don't contain the types of articles you thought you'd find, take a step back and skim them again with an open mind. Instead of starting over with a totally new topic, use the evidence you've discovered to adjust your topic and create a new question that is answerable in the literature.