Charles and others also noted that rising markets generally have made it more difficult for fund managers to find attractively priced businesses.
One factor is the strong market for mergers and acquisitions. Companies themselves have plenty of money to invest, which creates competition—and higher prices—for traditional PE fund targets.
"The broader M&A market continues to make life difficult," PitchBook said in a recent note on industry trends.
PE funds have responded by changing how they invest.
Managers are moving away from buying an entire business—a so-called platform buyout. Instead, they are adding investments to existing portfolio companies—an "add-on"—or taking a minority stake in a growing company.
There were 359 platform buyouts in the first quarter, according to PitchBook, the lowest number since the fourth quarter of 2009.
Add-ons include mergers and acquisitions, where a company backed by a PE firm buys acquires another.
A recent blockbuster was the announcement that Kraft Foods Group would combine with H.J. Heinz. Brazilian PE firm 3G Capital bought Heinz with Warren Buffett's Berkshire Hathaway in 2013. The combined firm will go public as part of the deal; Heinz shareholders will own 51 percent of the company following the more than $40 billion deal.
Read MoreBuffett's HJ Heinz to merge with Kraft Foods
"Private equity fund managers are evidently finding greater value in combining portfolio companies within certain sectors, particularly as the global economy begins to stabilize and competition intensifies within specific markets," Christopher Elvin, head of private equity products at Preqin, said in a report Wednesday.