Nine months after Americans began signing up for health insurance under the Affordable Care Act, a challenging new phase is emerging as confused enrollees clamor for help in understanding their coverage. Nonprofit organizations across the country are being swamped by consumers with questions. Many are low-income, have never had insurance and have little knowledge of the health-care system. The rampant confusion poses a potential hurdle for the success of the health law: If many Americans don't understand how health insurance works, that could hurt their ability to use their benefits — or to keep their coverage altogether.