I am not sure if you typed this wrong in your question or if this is in your profiles.
<ref name="1851 census"</ref> should be
1st time - <ref name="1851 census"> census citation </ref>
2nd time using that same citation, it would be <ref name="1851 census"/> same name as above, but with slash inside and no ending 'ref'
Help Page for Suggestion 872
To make it a little easier on yourself, you only need quotes around the ref name when you are using a space or start with a number, so if you start using C1851, as an example, that will make it easier. C standing for census and no quotes are needed
I have found about 15 or 20 different problems that have caused this new Suggestion, mainly syntax issues, like the slash is in the wrong position, extra spacing around the = sign.
Hope this helps.