Too few or no results:

Reconsider the number of concepts included in your search. You may need to search for fewer individual concepts. Also, think of other keywords for your concepts that might help retrieve results.

If a few results are somewhat on topic, take a look at the additional information for those sources. This information can generally be viewed by clicking on the title. Many databases assign subject terms to each source. Consider whether any of these subject terms, or other terms found in the abstracts/summaries, might serve as new keywords for further searching.

Check your spelling to make sure your keywords are correct.