Burned-out employees are often easy to spot. But what about people who some workplace experts describe as "rusting out" on the job?

A survey by scheduling software provider Deputy of hourly workers across the hospitality, retail, healthcare and other sectors found that many people are languishing in their jobs and are functionally less productive than their employers expect. 

Roughly 15% of those polled reported feeling merely "OK" about their jobs, a sign that researchers say can lead to lackluster performance, Star Levandowski, senior vice president of business and people operations at Deputy, told CBS News. "We see that as an early indicator to rust out, when people are not feeling challenged enough," she said.

What is rusting out?

Think of a mechanical tool that's shiny and sharp when new, but that can get rusty over time if left hanging unused. 

"If you feel like you're not being used to the best of your abilities, you feel like you're just hanging out, or rusting out," Levandowski said.

Signs of that feeling on the job can be subtler than indicators of burnout — the more commonly understood issue of mental and physical exhaustion that may follow long periods of overwork and pressure to perform. 

"With burnout, you see people become really disengaged, and the result becomes quite obvious," she said. 

But workers who are rusting out still report to their shifts and perform, although with a fading sense of purpose. They might also feel fatigued because their manager hasn't shown interest in their development. 

"They still come in and do the work. But it's a function of people not being stretched enough, feeling challenged or heard," Levandowski said. "When that happens, and you don't get the most out of your workers, your business won't perform as well."

Edited by Alain Sherter