Apple on Wednesday reported the kind of quarter most big companies would envy, posting a profit of $13.1 billion and selling 28 percent more iPhones and 48 percent more iPads, its two biggest products.

Its stock sank 11 percent Thursday.

What is going on? Because of its great success in recent years, many investors have come to expect nothing short of perfection from Apple. And while it is still widely considered the most innovative company in the technology world, a maker of products that its devoted customers cannot live without, Apple is facing a range of challenges.

It is dealing with increased competition from big rivals like Samsung and Google, and with so many people already using smartphones, the market is not quite as untapped as it once was. Apple is forging into cheaper product categories, meaning lower profit margins. And given that Apple has grown so big, with sales of more than $160 billion in the last 12 months, keeping up its heady growth rate is becoming harder and harder.

Once-euphoric investors, who pushed Apple’s stock to a record high of $702.10 last September, have become nervous, and in early trading on Thursday, the stock traded at $455.89, down more than 34 percent from its peak.