I found myself going through the 1940 Census on Ancestry.com trying to figure out why on earth my Grandfather and the rest of my mother's family had seemingly disappeared during that Census. After getting very frustrated in not finding them where I knew they should be living, I turned to another resource - FamilySearch.org. I performed the same search, and voila! The family appeared in the search and I went right to the Census sheet.
Hmm...something is obviously amiss here.
So now the question became, "Why does Ancestry not show them???" After browsing through the individual images of the 1940 Census for the enumeration district where I had confirmed they lived, I came upon the answer... The transcriber for Ancestry.com listed the last name as "Lisk" vs the correct name of "Link."
This is probably one of the most blatant examples I have run into of where transcription leads to a lost relative. Even the Soundex Codes for the two names are different - L520 vs L200. They even mis-transcribed "Hannah" into "Hanuah"!
Needless to say, it pays to not accept that a relative is "lost" just because a search doesn't yield the results you think it should. Try different avenues, and you may just find that someone else just "misplaced" your relative!