Shares of Green Mountain Coffee Roasters (GMCR) were down again Thursday after a research firm said sales of the company's K-cup pods hit a "noticeable slowdown" at U.S. offices last month.

The company's stock price fell 5.5% to $20.13 in afternoon trading, creating more pain for investors still holding on. Green Mountain shares have fallen more than 80% from September.

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It's pretty clear that this stock will never be the darling it once was unless there's a major change in strategy. Green Mountain hit the jackpot with its Keurig brewing machines and the innovative K-cups used in them.

But the competition is coming, and it's fierce and deep-pocketed. Green Mountain will never be the same again.

Here are three forces bringing Green Mountain down low.

Expiring patents. About 75% of Green Mountain's sales come from its K-cups, the little pods of coffee that brew one cup in the Keurig machines. And while the company has dozens of patents that cover nearly every aspect of the machine, two patents related to the cup expire in September. You can see the patents here and here.

K-cup competition. Even though the patents haven't expired yet, competitors are already moving in. Grocery giants Kroger (KR) and Safeway (SWY) are launching their own store-branded cups that work in Keurig brewers.

It's likely the grocery chains have a license from Green Mountain since the patents haven't expired yet. But those licensing fees could soon decline precipitously or disappear altogether, depending on the patent situation.

The question becomes, then, whether Green Mountain will be able to compete as more companies jump into this hot sector. Kroger and Safeway K-cups will undoubtedly be less expensive -- and perhaps less tasty -- than Green Mountain's premium brands. But they may be just the right price for people who don't want to pay Green Mountain prices, which can be as much as $1 per K-cup.

Why did sales hit a "noticeable slowdown" at U.S. offices? You guessed it -- offices are buying competing cups, according to the research report from Detwiler Fenton.

Brewer competition. Green Mountain has recently come out with its new brewing machine, called Vue. The $250 Vue system is pricey, but Green Mountain hopes coffee fans will love the Vue's expanded capabilities, including customized water temperature and brewing strength.

But the Vue will have some tough competition from Starbucks (SBUX) and Wal-Mart (WMT). Starbucks is debuting its own single-cup brewer, called the Verismo, later this year. The Verismo will be different from the Vue in that it's a high-pressure system designed for making espresso drinks. It will likely brew regular coffee as well.

And while you can find Starbucks coffee pods for the Vue, you won't find any Green Mountain pods that work with Verismo. Starbucks will only use its own coffee pods for Verismo machines. There is no price or launch date announced for the Verismo yet.

Wal-Mart plans to start selling the Esio single-serve drink maker this year, Reuters reports. Esio can make energy drinks and vitamin waters in addition to coffee and tea, and its prices are expected to be lower than Green Mountain's.

Green Mountain has had a nice ride that was relatively free of competitive threats. But that's all changing as well-heeled rivals introduce newer, cheaper products. Unless Green Mountain can strike gold again, this stock is done.

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