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Try to increase the kw density of this phrase on the page, without changing much else.
The SE's where you are performing less well rely more heavily on kw density and other on-page factors.
About 4 months or so ago one of the SEO Co's published comparisons for G,Yahoo and MSN for the number of words on high ranking non comp KW pages.
800 to 1200 is definitely the range you need to be in.
The trends were very strong and worth seeing.
Not sure is i can post the URL here, so you are welcome to email me directly