(Bloomberg) -- Tax season is supposed to be over on April 15. But among certain groups—especially the wealthy—filing for an extension until Oct. 15 is now routine.

In 2011, 11 million taxpayers filed for an extension; two years later, 13 million did, an increase of almost 20%. At the end of September 2014, more than 25% of those who had filed for an extension were still working on their filings. We're not just procrastinators. It has gotten harder to file on time. Here’s why:

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