I have a family in the 1861 census, but would like to go back a previous census. It seems to be missing. Can't find either the 1851 or the 1842 census for that area

It's possible those census returns did not survive. If you've checked the book "Catalogue of Census Returns on Microfilm, 1666-1891" published by the National Archives of Canada and it does not include a census for the area you're seeking, you're likely out of luck. The National Archives is the caretaker of most Canadian census records. There are a few census records out there that are not at the National Archives but in order to find these you will have to do some research. Write to the genealogy and history societies in the area to learn if there was in fact a census that survived or if there are alternative resources available.