I type many articles a day. I use long-tail keywords and usually rank on top basically no matter what for the keywords that I decide to use. For instance, yesterday I wrote an article titled, "Teeth Whitening Hydrogen Peroxide." Obviously, without the quotes. So yesterday, somebody on google found my site for that key term. Today, I type in the same key term and my site is nowhere to be found. Why? Does this have to do with the way google caches pages or what?
Because rankings fluctuate daily, and sometimes more frequently. The data that Google collects is constantly changing so it's not surprising that these changes occur, especially for new pages.