Recently I was searching the 1855 New York State census for relatives from Clinton County, New York, by querying an index. No amount of clever searching, Soundex options, and wildcards were effective. It was when I went to read the census manually that I realized the problem.
Clinton County’s 1855 New York State Census enumeration is not on Ancestry.com (where I initially searched). Reading the description of the 1855 New York State Census indicated Clinton was one of the counties whose enumeration is not extant.
No amount of database querying is going to find those Clinton County relatives in that census. Make certain the database is complete or at the least includes your probable areas of ancestral residence before searching.