A subject database will almost always cover more journals and cover different dates in your field than does Web of Science (WoS), so it will often be possible to find additional citations. In addition, one simply can't rely on the completeness of coverage in Google Scholar.
Note: Many databases have only fairly recently begun to track citations.
The following databases support cited reference searching:
Academic Search Complete - EBSCO Search Tips
Alt-Press Watch - ProQuest Search Tips
America: History and Life - EBSCO Search Tips
Historical Abstracts - EBSCO Search Tips
JSTOR - JSTOR Search Tips
ProQuest Central - ProQuest Search Tips
Includes the following databases:
Dissertations and Theses