Sometimes They Lied
Believe it or not, sometimes our ancestors lied. There are numerous reasons they felt some family secrets must be kept forever. Out-of-wedlock births, Indian blood, suicides, bankruptcies, retarded children, mental illness, and criminals in the family are often hidden in a variety of ways.
In one instance I heard of a thief who was arrested and sent to jail for several years. His family marked his death in the family Bible as the day he was convicted. No mention of him was made again in that family. His descendent, however, read about his circumstances in an old family letter tucked in the same Bible. He was persistent and searched prison records until he found him. He then was able to track him when he left prison. His ancestor had Americanized the spelling of his last name, remarried and had a new family.
You may have to do a lot of digging to turn up clues and facts about a hard to trace family member. Look everywhere. Census records, church baptismal records, school enrollment, draft registration, military records and just about anywhere they kept a list of names. Never give up, there could be a new clue around the next corner.