Styles not being applied to editor

We are having an odd issue with our install of CKeditor (4.4.0), where the styles in contents.css aren't being applied to the content of the editor.

CKeditor is obviously reading the stylesheet, as the Headings under the "Paragraph Format" dropdown are styled correctly, however none of the styles are being shown in the content of the editor, and it is just following the default styles of the web page that the editor is installed on (as opposed to the declared styles in contents.css).

We have tried delcaring custom stylesheets, using @import, and a couple of other ways to get our styles into CKeditor, but to no avail.

I tried uploading an older version of CKEditor (4.2) with our same config setup and all worked fine.

Has anyone else come across the same problem?

Thanks!