Warren Buffett has carved out a core stock-picking strategy of investing in companies with strong economic "moats," businesses that have built, fortified and generated success from well-known brands that make it difficult for them to succumb to competitive forces.

But for a number of holdings in his stock portfolio, the moats may be drying up and the walls could be breached.

Stalwarts like International Business Machines, Coca-Cola Inc. and Procter & Gamble Co. have shown declining revenue trends in recent years, and face competition that may make it more difficult for them to outperform the market in the way they did in the past.

On average, the 15 biggest positions he owned at the end of 2014 have gained 7.8 percent in the last 12 months, compared with a 13.1 percent rise for the Standard & Poor's 500 index.

Buffett supporters — and there are many — would say that the 84-year-old investor is hardly looking at the short term, but what stands out about some of the larger holdings are weakening revenue trends that augur concern about the long-term, not just the short term.

"The moats are not as deep and impenetrable as in the past," said Doug Kass, who runs Seabreeze Investment Partners in Palm Beach, Fla., and has questioned Buffett's investments in the past. He currently has no position in the stock.

That is not to say Buffett is any kind of a slouch. Those 15 holdings, over the past five years, on average, are ahead of the S&P, with an average gain of 85 percent, compared with the S&P's 78 percent rise.

And he can still be opportunistic, as in the aftermath of the financial crisis when he acquired a $750 million position in Goldman Sachs Group Inc. that's now worth $2.5 ­billion, and an option to buy 700 million shares in Bank of America Corp. for $5 billion — now worth $11.2 billion.

What's less clear, however, is whether these previously unassailable franchises are facing competitive pressures that will continue to hurt sales growth. Buffett owns 76.9 million shares of IBM that as of last year had cost him $13.2 billion, and that position is underwater; IBM is trying to reverse 12 straight quarters of year-over-year revenue declines as it plays catch-up in the cloud ­computing space.

Two other large positions, Coca-Cola and Wells Fargo & Co., are also struggling. Wells Fargo has had three consecutive quarters of year-over-year revenue growth, but that followed 18 quarters of decline. Revenue growth has fallen for Coke in eight of the last nine quarters as the company deals with changing consumer tastes.

But of course, Buffett's total cost for his 400 million shares of Coke was $1.3 billion. As of Wednesday, it was worth $16.2 billion.