I've just spent a couple of hours looking for people on the 1861 census - both on Ancestry and on FreeCen. There are several people I'm missing from that year - in fact a large proportion of the people in my family tree who were around then! I belatedly bothered to check how good the coverage of Lincolnshire is on FreeCen and found it isn't anywhere near complete! I suspect spelling errors or transcription errors are the problem with the ones I can't find on Ancestry. I think 1861 is the year I've had most problems with so far. I also suspect the Ancestry site was busy today because of the records of Old Bailey trials being newly available.
Looking on the bright side I've ordered a copy of the birth certificate for a half sister of my great grandmother because I want to see who her mother was and therefore who my great great grandfather's second wife was so that will make interesting reading when I get it.