If you have found a census record on FamilySearch but not on Ancestry it is probably because FS uses the FMP transcription and Ancestry have mistranscribed or missed the relevant entry. Annoyingly, FS does not provide the actual census reference (only its own film reference), but you can use the enumeration district and subdistrict information to browse through the relevant pages on Ancestry. Alternatively, try searching without a name (or with either forename or surname only) but using the age, gender and location information provided by FS.

Ancestry only have a fraction of the parish registers available via FS, but if you know that the registers concerned *are* definitely part of Ancestry's collection, try browsing rather than searching.