The Dow Jones Industrials (DJINDICES:^DJI) climbed above the 16,000 mark for the first time this week, with the average taking just over six months to go from hitting the 15,000 milestone for the first time to its latest achievement. But four names stand out for posting the biggest negative returns over that time frame: IBM (NYSE:IBM), Caterpillar (NYSE:CAT), Coca-Cola (NYSE:KO), and AT&T (NYSE:T). Let's take a closer look at these four stocks to figure out why they made investors wait longer before the Dow could hit 16,000.

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IBM has fallen almost 10% since the Dow first hit 15,000 in May, and with its high share price, the tech giant has been a big drag even as the Dow has overcome IBM's downward pull. The company has done a reasonable job in maintaining profit growth, but revenue has been harder to come by, with big drops in sales in its systems and technology business as well as its global technology services division. Emerging-market economies have also hurt IBM's growth, with a 12% drop in revenue from Brazil, Russia, India, and China in its third quarter. CEO Ginni Rometty still believes that high-growth areas like cloud computing, mobile, and enterprise data analytics hold the key to future success, but the company will have to work hard to meet its goal of reaching $20 in operating earnings per share by 2015.

Caterpillar dropped more than 6%, continuing its swoon from earlier in the year. The heavy-equipment maker has suffered greatly from slowing growth in emerging markets, as construction and infrastructure activity starts to ramp down from previous levels. Moreover, the plunge in commodities prices earlier this year hurt its mining-equipment demand, and cash-strapped mining companies are unlikely to spend as much on capital expenditures as they did when prices were soaring. When the business cycle moves toward expansion, Caterpillar should bounce back, but it's uncertain how long it will take for that to happen.

Coca-Cola has fallen 4% since May, as the beverage company faces the challenge of transforming its focus away from its namesake carbonated soft drinks toward higher-growth areas. Coca-Cola's recent results reveal that obesity concerns have hurt soda consumption both in developed countries and in certain emerging markets as well. On the other hand, demand for non-carbonated water, juice, and energy drinks has been relatively strong. The latest threat involving potential health impacts of diet sodas using aspartame shows the magnitude of the difficulty that Coca-Cola will face if customers buy less Coke and Diet Coke, which are now the No. 1 and No. 2 soft-drink brands in North America.

Finally, AT&T has seen a 3% decline in the past six months, largely because of the rise in competition from its rivals. Not only does the No. 2 U.S. wireless carrier face a stronger Verizon Wireless division unified under a single owner, it also has to deal with the emergent prospects for the No. 3 and No. 4 carriers in the market. Even promising sales of new smartphones haven't turned things around for AT&T, and until it makes its intentions clear on how it expects to boost growth going forward, investors will remain wary.

All four of these stocks have solid long-term prospects, making it likely that they'll turn around eventually. But they also have to overcome the obstacles in their paths before they can join the Dow by moving higher.