Everyone's got an opinion on buybacks, from executives and politicians to Twitter users. Now, in a year when it was once thought they'd reach $1 trillion, a louder voice is being heard: the market's.

While the efficacy and even purpose of buybacks are the subject of endless argument, if you believe they're meant to enhance returns, lately they're falling short. Baskets of stocks with the highest cash-return rates have been trailing the market in recent months.

Companies may have noticed; repurchase rates are falling. Maybe it's that valuations are stretched, though price-earnings ratios were higher a few years ago. As for buybacks losing their bang, that could reflect changing tastes for how companies use cash. Anything from a growing preference for capital investment to aversion to piling on debt to pay for them.