After going through tough times last week, the Dow Jones Industrials (DJINDICES:^DJI) were having a happy Monday morning, with the average gaining more than 133 points by 11 a.m. EDT. Investors celebrated signs of health in the U.S. economy, as retail sales soared by 1.1% in March. That was the largest gain since late 2012 and supported the hypothesis that sluggish economic activity during the winter was merely an aberration. Yet despite the bullish mood, UnitedHealth Group (NYSE:UNH) and JPMorgan Chase (NYSE:JPM) fell modestly.

UnitedHealth Group's 0.24% decline comes as the last stragglers from the Affordable Care Act's individual health insurance mandate start coming into the system. Yet even with Obamacare now having taken full effect, UnitedHealth and its peers are still discovering new quirks in the system. Over the weekend, The Wall Street Journal reported that many patients are responding to higher deductibles and co-pays by trying to work existing health problems into annual physicals, which are deemed to be preventive in nature and therefore are generally available free of charge once per year. Given the complexity of Obamacare, UnitedHealth Group can expect patients to try to take advantage of this and similar provisions, and the health-insurance company will have to stay on its guard to preserve profits in the face of potential challenges from lower reimbursement rates and patients with pre-existing conditions coming onto its insurance rolls.

Meanwhile, JPMorgan Chase dropped 0.8% after a number of analysts reduced their price targets on the bank stock. In light of last week's disappointing earnings, JPMorgan Chase clearly faces challenges to its future growth, and this morning's more favorable earnings report from one of its main big-bank rivals shows just how much ground the company must make up in order to compete effectively. One key question JPMorgan must answer is whether its integrated investment banking and consumer banking model will work in the long run, or whether it will need to make strategic shifts in one direction or the other to maximize its growth potential and move forward in a much more tightly regulated environment.

It's not unusual for some stocks to lag when the Dow rises, but it's important to keep your eyes on why. If shares fall behind the stock market too long, it can show signs of weakness that you should heed rather than ignore.