Have you ever checked an index and not found the person you were looking for? It is a common enough occurrence. This can happen in the census, ship records and just about anywhere else where records have been indexed. Do you accept that your ancestor is not in that record based on the index? The answer is a resounding NO.
Indexes are created by people. Mistakes happen. Names are misspelled, misread and just plain missed. It happens and much more often than you would like to think. Often these indexing projects are handled by people who may not be familiar with local names and spelling variations. I can think of several I have come across in recent research.
Here are three examples from my own research. In the Irish parish records index on RootsIreland I could not find my ancestors. However, when I manually went through the parish records they were there. Moran had been written as Horan. In England, Ollard was written as Holland. In the 1940 census Dimingos was writes as Domingues. If I had not taken the time to look at the actually record not just the index, I would never have found them.
Use an index as a guide but if someone isn't where they should be, take the next step and look at the original record.