Asking an answerable research question...and making your database searching much easier in the process.
It is important to choose your search terms/keywords carefully. Is there more than one term for the concept for which you are searching? Are you using a technical term or the layman's version? For example, if you are researching breast cancer, it is also known in the nursing literature as breast neoplasms. Likewise, Primary Care may be in an article as Primary Health Care, and by searching the former, you could miss articles that use the latter. To avoid leaving out relevant articles, use all of the possible terms for your concept:
"breast cancer" OR "breast neoplasms"
"primary care" OR "primary health care"
Alternatively, you can search using subject headings. All databases have a list of controlled vocabulary words that seek to apply a single term to a single concept. When you use subject headings, you do not have to brainstorm all of the possible synonyms for your search term, because the database does that for you. When you enter a serach term, CINAHL and Ovid are set up to automatically suggest subject headings. For instance if you search breast cancer, CINAHL will show you a list of subject headings, of which breast neoplasms is the first result. Try searching with subject headings and see how your results differ from searching with just keywords.