Start by reviewing the search results to determine if there is a need to improve or change your search. You will probably need to revise and change your search several times before you are satisfied with the results.
If you use the advanced search form in a database, you have more opportunities to improve your search. By choosing advanced search you can control in which fields the database should search for your search terms. For instance, you can decide to only search for the search terms in the title or abstract field.
Are you getting too few hits?
Are you getting too many irrelevant hits?
Do you get hits that are irrelevant to your search question? This is completely normal especially when conducting systematic reviews. It does not mean that there is anything wrong with your search. No search can only capture relevant publications without missing other relevant material. The most important thing is that the search captures a great amount of relevant hits and that the total amount of hits is manageable. One way to review your search is to test whether any of the articles you have already found are included in the result list.
Read more about how you can improve your search using different search techniques.