McDonald's (MCD) has shown an impressive knack for survival over the past few years as the economy has gone south. When other rivals stumbled, McDonald's roared. It knew exactly how to manage the turmoil and come out ahead.

But something has changed. The company just had its worst performance in more than nine years. Sales fell across the board in July, and the numbers were below the expectations of analysts and even the company itself.

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Same-store sales -- a key indicator of performance because it measures sales at established restaurants -- came in flat. Only two weeks ago, McDonald's said it expected those sales to rise, Reuters reported. Analysts were expecting a 2.3% increase.

Shares fell 1.7% to close Wednesday at $87.53. Shares are down 11.5% this year, and one of the basic tenets of investing -- that McDonald's is a solid, dependable industry leader -- is now being questioned.

"We've grown used to seeing McDonald's weather bad economies, so this is a bit of a surprise," RBC Capital analyst Larry Miller told Reuters.

Another analyst, Sara Senatore of Bernstein, says, "McDonald's may be underperforming the industry, which is not typical for them," she said, according to The Associated Press.

Why is McDonald's suffering? Here are several reasons, according to AP and Reuters:

1. Intense competition. Just look at Burger King (BKW), which has David Beckham as a star endorser and has also begun offering fruit smoothies and salads. Then there's the breakout Doritos Locos Taco from Taco Bell, owned by Yum Brands (YUM). Wendy's (WEN) has also been improving its advertising and its lineup.

2. Last July was good. McDonald's debuted the mango pineapple smoothie a year ago, which was great for business. It's harder to compete this year.

3. Problems in Europe. This comes as no surprise, but analysts still expected the company to see a 2.4% sales increase in Europe as the troubled economy led consumers to trade down to fast food. Instead, McDonald's said its July sales in Europe fell 0.6%.

4. Flooding in China. McDonald's also disappointed in the Asia, Middle East and Africa regions, reporting a drop of 1.5% in same-store sales. Analysts expected to see a 1.4% gain. Extreme flooding in China was partly to blame, as was a change in the timing of Ramadan and continued problems in Japan.

5. Disappointing response to McBites. McDonald's stopped promoting its chicken McBites as heavily in the U.S., and sales of the new item tumbled more than expected as a result.

6. Calendar shift. This year, there was one less Friday and Saturday in July. That makes a difference.

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