Mistakes happened at every stage. Someone may have been somewhere unexpected, missed out completely, or entered with a distorted name. They might have been entered correctly but the record has gone missing, or has been transcribed incorrectly.
As far as I know, all surviving Census records (1841-1911) have been transcribed and are available online. Some of the providers have missed bits here and there but between them they've probably covered all there is. If you can't find someone on the site you subscribe to, you can usually search on other sites without paying. You won't be able to see all the details but can usually see enough to tell if they have a record your site doesn't. Some records have not survived at all or are illegible to some extent. The best known is a large chunk of of the Manchester 1851 records which were damaged by flooding, but odd pages have been lost from all over.
And yes, you can sometimes find someone with the right name in the right place and yet it's not the right person. Even if it's a very rare name, he or she could still be a cousin of the one you want.
If the person you can't find is George's putative father John in Lympsham in 1841, Ancestry has him as John Bown. The original entry was probably intended to be John Bower.
David
