Singular / Plural | Thread profile

Hi, I've noticed that one of our biggest keywords suddenly dropped from 2 to about 30 in Google. However the plural form of the same keyword is still where it was (position 2). I don't understand why this has happened since I thought Google stemmed everything. Has anything like this happened to anybody else recently? Shall I change the content of the page to mention the singular more often...

Have your conversions (sales, leads, signups, whatever) also dropped off? What about your overall site traffic? Rankings are nice, but you pay the bills with sales. So don't go changing things all around to chase rankings unless there's also a negative impact on the things that matter. Could be the singular of the phrase doesn't convert well, so a loss in rankings there won't really make ...

Thanks for your response. The conversion rate for both terms is around the same but the singular brings in more traffic (or at least it did). Checking by hand or using my ranking software (which I think screen-scrapes rather than uses the API) I've seen the keyword has been way down for a few weeks now. I appreciate that rankings are not everything and that is important not to get too ...