Apple (Nasdaq: AAPL) has tumbled 20 percent since reaching an intraday high on Sept. 21, entering what’s known as a bear market as the company faces accelerating competition and struggles to make enough iPhones to meet demand.

Amid broader market declines, Apple fell 3.8 percent to $558 at the close Wednesday in New York, after accounting for a dividend payout of $2.65 a share. The stock peaked at an intraday high of $705.07 on Sept. 21. A bear market is generally defined as when a stock falls 20 percent or more over a prolonged period.

Apple, the world’s largest company by market value, hasn’t been able to keep up with demand for the latest version of the iPhone, which accounts for about two-thirds of the company’s profit. Terry Gou, the chairman of iPhone-manufacturer Foxconn Technology Group, said today that its assembly plants are “falling short” of demand.

New entrants also are challenging Apple’s dominance in the market for tablet computers. Apple’s market share dropped to about 50 percent in the third quarter from about 66 percent in the prior three-month period, while competing products from Inc. and Samsung Electronics Co. gained, according to market researcher IDC.

“Some of the patina has come off the Apple story with regards to the perfect execution they’ve been known for,” said Alex Gauna, an analyst at JMP Securities LLC in San Francisco. “Expectations got high for this company and it’s getting harder for them to meet.”

The stock slide comes during a critical period for Apple. The company refreshed nearly its entire product line in time for the holiday shopping season, from the least-expensive iPod media players to top-of-the-line Mac computers. Chief Executive Officer Tim Cook also introduced a new iPad with a smaller screen to fend off Amazon, Google and other rivals. Apple said last month that the product releases would bring down profit margins.

The fiscal first-quarter, which ends in December, is typically Apple’s strongest period. Analysts are predicting the company will earn profit of $12.9 billion on record sales of $54.5 billion.

There has also been upheaval within the company’s leadership ranks. Cook last month overhauled Apple’s management team, with longtime iPhone and iPad software chief Scott Forstall and retail head John Browett exiting.