Hi jess
They were in Preston on the 1851 census and they were in Preston on the 1871 census and I suspect that they are also there in 1861.
However in 1851 on Ancestry the enumeration district which contains them has 13 pages in total. In 1871 it has 10 pages in total. In 1861 the same enumeration district only contains 3 pages (and one of them is invalid).
You can see this if you look at
RG9/1900 Folio 155 page 1. There are only a few entries (including the Griffiths family who appear as neighbours of the Pritchards on each of the other censuses).
There are obviously several pages not showing up for this district in 1861 on Ancestry.
I had a look on Family History Online but the 1861 census is not available at all on there.
Sorry

DS