How can I make the split bibliographies work?

Question

I am using Windows 8 64 bit, TeXStudio and MiKTeX, and I am trying to implement the following code in my work:

How to split bibliography into "works cited" and "works not cited"?

Now, biblatex works just fine and everything, but I am having some trouble splitting my references in a cited and a non-cited part.

My problem is the following: Let's say I have two items in my bibliography. When I cite the first item, it shows up in the references part, but the second item does not show up in the “further reading” part. Now when I cite the second item and compile, it does show up in the References part. When I then remove the citation of the second item in the document, and then compile twice, it does show up in the further reading cathegory. Summary:

\cite{article1}

References article1

Further readings empty

After that:

\cite{article1}
\cite{article2}

References article1 article2

Further readings empty

Then:

\cite{article1}
%\cite{article2}

and compile two times:

References article1

Further readings article2

Isn't it strange? Of course I would like if I wouldn't have to go through this procedure, as I have a bibliography of more than 40 items, still growing. I should be able to compile the last configuration directly, and that then article 2 would show up directly under Further readings. ;)

I am using:

\usepackage{filecontents}
\usepackage[defernumbers=true]{biblatex}

\DeclareBibliographyCategory{cited}
\AtEveryCitekey{\addtocategory{cited}{\thefield{entrykey}}}


\begin{document}

\printbibliography[category=cited]

\printbibliography[title={Further reading},notcategory=cited,resetnumbers=true]

\end{document}

What I think might be the problem, is that \AtEveryCitekey and \thefield are marked as unrecognized commands in TeXStudio.

Do you know what might be the problem?