Researching family history is fun and rewarding.
It can also be very frustrating at times, especially when some relative you are researching, and whom you know existed, just seems to "disappear", presumably without trace. When this happens, and it will, just as it happened to me on several occasions, leave it alone for a while.
Try to think of another approach to this person, perhaps through another branch of the family, or another research site.
If at all possible try and find paper records at the relevant local authority depository, or even the Parish church in the area where the person resided.
One of the reasons why this happens is that most of our research is conducted via the Internet and our trusted computer. But we are then very much dependent on the original source material being input correctly, and with the right search "markers" attached. This does not always happen one hundred percent, we are only human after all, and people sometimes do make mistakes. Then, one day the site managers rearrange how data is addressed, or you use a different search "marker", and up pops the information you have been looking for.
Another more common reason relates to the source document, which in many cases is the original birth certificate, marriage certificate or census paper. Many of the working class people, even in the 19th century, could not read or write. Often a name would be misheard, or mispronounced, and when written incorrectly by the registering authority, priest or census enumerator, could not be corrected because of the illiteracy mentioned.
It was also common practice during the early censuses to round peoples ages to the nearest five years. This means that data could be incorrect by at least two or three years.