For FAQs and current status of services and policies see UW Libraries Operations Updates
No single database or search engine is complete.
Different cited reference searches in different resources yield different results. For example, see What About Google Scholar?
PubMed's cited reference feature provides links to citing articles found in the PubMed Central subset of PubMed.
PubMed Central is a repository of publicly available full-text articles that have resulted from NIH funding or that have been deposited by approved publishers.
Look at the abstract view to see PubMed Central articles citing your article of interest
Databases on the EBSCO platform, such as CINAHL, PsycInfo, and Academic Search Complete include links to citing references as part of the references. Clicking on "times cited in this database" take you to the citing references.