During most of the 19th century, and into the 20th century, there was no universally agreed way of spelling Berkhamsted - or Berkhampstead and other variations were not uncommon. For instance the official name for civil registration was Berkhampstead up to 1939 when it became Berkhamsted. The FindMyPast censuses names the town (when doing place name searches) as
Berkhamsted (1851, 1861, 1891, 1901)Berkhamstead (1851, 1871, 1901, 1911)Berkhampstead (1841, 1861, 1871, 1881, 1891, 1901, 1911)
These differences appear to have arisen because the people carrying out the census did not agree the name of the registration district they were recording! FindMyPast does not allow wild cards when searching for place names and this means that you will find no one living in Berkhampstead in 1851 - which was the official name of the registration district. In four census years a search for people living in Berkhampstead only some of the people living in the registration district will be found.
There seem to be similar problems with Ancestry but I did not carry out full checks.
I work on the basis that all online searches could be misleading and if I search for someone and do notget any responses I open up the search to, for example, see if there is anyone called John (any surname). If I still get nothing I try and find out whether there are relevant gaps or systematic errors in the data base - or the search routine is loosing the data,