Duff is one of more than 40 million Americans who attended college but left without earning a degree or credential, according to the National Student Clearinghouse Research Center. Since the pandemic, the number has grown. Talk to college presidents facing declining matriculation out of high school and a looming demographic cliff, and they’ll point to people like Duff as key to raising enrollment. After all, they have already shown an interest in college, and many have credits that would put them closer to earning a degree. Many colleges see these potential students — call them dropouts or stopouts or comebackers or potential completers — as an easy win. It may not be so simple or straightforward, however. The reasons that students leave college are many and complicated. Some find themselves academically unprepared, laboring to keep up with their coursework, while others can’t shoulder the costs. Some are directionless, racking up a hodgepodge of credits with no clear idea of what to study; others flounder socially or struggle with mental-health challenges.