Some online databases are "works in progress." Ancestry.com, Footnote, GenealogyBank and other sites offer wonderful data, but some databases are not complete before they are posted.
Ususally this information is somewhere on the site, but it may not be obvious initially. Footnote.com is pretty good about showing users their "green status bar" that indicates what percentage of records have been uploaded. Ancestry will usually show what areas and records are in a database, but one has to scroll past the search screen to get to it.
FamilySearch also indicates when databases are incomplete.
Make certain you know how complete something is before searching it.