Andy_Langton - 7:57 pm on Apr 18, 2012 (gmt 0)
Robert Charlton is exactly right - look at the ranking factors affecting the two different queries.
I'm in the UK myself, and so I've found that US-spellings in the UK market are not treated in the same way as the local spelling. Similarly, searching in English at regional Google's will return results handled very differently from the competitive keywords at those engines - i.e. the keyword written the way the local market would use it.
If backlinks have played a part in your historic rankings, that's a good place to start digging.