Starbucks sells 4 billion paper cups annually, and most of them are never recycled.

"Recycling seems like a simple, straightforward initiative — but it’s actually quite challenging," the company said in a statement, explaining why it was failing to reach a seven-year goal of implementing recycling programs in all of its stores.

In a post on BloombergView, writer Adam Minter explained why the world's largest coffee chain can't figure out how to recycle.

1. The cups have a plastic lining that prevents leakage, but is difficult and costly to remove. "Recyclers will only bother [to remove the lining] if they are supplied with enough used cups to justify running the process on a regular basis," Minter writes.

2. Starbucks can't provide enough cups to justify the costly recycling process. A pilot program in 2010 collected three tons of cups from 170 Toronto stores. But that was only a tiny fraction of the 51.5 million tons of recyclable paper products collected in the U.S. that year, according to Minter.

3. Composting isn't a great option either, even though it would keep the cups from ending up in landfills. Composting "generates greenhouse gases while destroying the recycling value packed into the cup’s fibers," Minter writes.

4. Starbucks has failed to get customers to start using personal reusable cups. The company set a seven-year goal of serving 25% of beverages in reusable cups by 2015. In 2011, just 1.9% of beverages were served in those cups.