@Japes: did you mean the description field in calibre, or a description inserted as part of the content of the epub? I understood your post to mean the latter, and in that case, it might be malformed xhtml in that particular file. Open a copy of the epub in Sigil, let it fix any errors, and run validation. I've had entire chapters disappear from a commercial epub on conversion due to bad xhtml.
If it is indeed the description field in the metadata, listen to Sabardeyn instead
Edit: my brain is obviously not in proper working order this fine morning. Modify Epub does probably not generate invalid xhtml, so scratch that. Still, an inspection in Sigil might uncover other problems with the epub itself.