URL doesn't rank unless I click on "omitted results"

[proboscis]

I had a page that was showing up in the number 15 spot for a popular 1 word search term and now it doesn't show up at all unless I go all the way to the end where it says: "In order to show you the most relevant results, we have omitted some entries very similar to the 932 already displayed.If you like, you can repeat the search with the omitted results included."

So I repeat the search and then the page is back at number 15. What causes this?

[Marcia]

It's probably being found "too similar" to other pages. Check for similarities with other pages on your own site, and definitely check for sites that may be COPYING your pages.