Hit Highlighting - Inconsistent

I Have SharePoint Server Standard 2010 and am using the core results web part on a number of pages to display results. The hithighlightingsummary property works sometimes and not others. The search results themselves are correct, the term searched for always appears in the document.  The data being searched are sometimes large files and I wondered if this had anything to do with it. 

I have not been able to find any consistent reason for why sometimes the hit highlighting works and other times it doesn't

I found what appears to be a similar thread on the forums here but unfortunately no explanation of the behaviour.

http://social.technet.microsoft.com/Forums/en-US/sharepointsearch/thread/4709904e-4180-46ad-81e7-5bf844304621/

Any help gratefully received

Jo