The series of Assize calendars is very incomplete at first. It starts with a few isolated calendars for prisoners held at Warwick Gaol (Summer 1801; Lent 1809; Lent 1829; Lent 1839; Lent 1841), but from 1849 to 1857, 1869 to 1871, 1874 to 1881 and 1888 to 1900 most calendars survive and have been indexed.
The calendars of Quarter Sessions prisoners are practically complete from 1800 to 1900, but the index does not at present cover the years 1840 to 1846 or 1879 to 1884 (with the exception of Epiphany 1879 and Midsummer 1883).
the above was taken from the website explaining about the prisoner records - so if your ancestor was in prison in 1871 when the census was taken - but he was up in court say 3 or 4
years previous, there are missing records between 1857-1869 - so this could well be why you can't find him.....
