Swift Transportation’s 20,000 workers haul goods in almost 14,000 big-rig trucks that travel the interstates and back roads of the United States every day. The company’s performance is closely tied to the nation’s economy, which has been looking increasingly sunny lately.

So it was surprising last month when Swift’s stock plummeted nearly 18 percent in a single day. The tumble came for an odd reason. It wasn’t because there was too little business — but rather, too much.

“We were constrained by the challenging driver market,” the company said in its quarterly earnings announcement. “Our driver turnover and unseated truck count were higher than anticipated.”

In other words, Swift had plenty of customers wanting to ship goods. But in a time of elevated unemployment, it somehow couldn’t find enough drivers to take those goods from Point A to Point B. How is that possible? The reasons for that conundrum tell us a great deal about what has been ailing American workers and why a full-throated economic recovery has been so slow in coming.