If you are preforming text searches of material that has been digitized, consider that the last name might have been hyphenated.
I searched for dingman in a digital version of the Sargent genealogy on Google Books. The only reference was in the index, not the book itself. That's because the actual reference in the book had the word Dingman split across two lines--hyphenated in the middle.
Is a hyphenated name giving you problems?
Some search sites may have taken hyphens into account when creating the OCR index.