As Frontier Airlines deals with customer complaints regarding Monday's delays at DIA, 9NEWS has learned that what the airline called "gate issues" was connected, in part, to employees from company headquarters being asked to help the ground crews.

Over the weekend, Frontier made the transition from one third-party vendor (Swissport) to another (Simplicity) to manage its ground operations. The company cut ties with Swissport at DIA, according to Frontier spokesman Jim Faulkner.

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"Swissport had some staffing issues over the summer and so, Simplicity's been our partner at other stations and we worked with them and that's why we decided to change, because of the issues with staffing," he said. "We've actually just hit a couple of bumps over the past few days. For the first part of September, on time arrivals were actually ticking upwards pretty well. We're just thinking this is a very temporary bump."

While half of the new grounds team for Simplicity is made up of former Swissport employees, the other half will have to undergo on-the-job training -- meanwhile, the grounds crews will be supplemented by workers from Frontier's corporate offices for at least another week, Faulkner said.

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"So as they go through their on-the-job training, we're sending out people from our headquarters to help supplement," he explained. "The corporate or the people from headquarters are going through training as well, and then the people -- the new-hires from Simplicity -- have to do it."

In addition to the transitional issues, Faulkner said weather and maintenance also played parts in delays.

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"For example, yesterday we had bad weather in Chicago. We have a pretty large operation there. And so what happens in Chicago can impact Denver," he said. "Also issues with maintenance, once in a while, you know you might have a maintenance issue and that could affect an entire line of flying."

Mac Clouse, a finance professor at the University of Denver, said it's not an ideal situation for customers, but it's a necessary move.

"Well, they've gotta do something during that transition period. And unfortunately, until that transition can take place, you have to put somebody in there. So you're putting people in there that maybe know about the job, but htey've never done it, so there's going to be problems associated with that," he said.

Though Clouse admitted, the use of third-party vendors on behalf of airlines continues to be a common trend.

"It's done a good thing for their profits. They've turned things around from a profit standpoint, where they were losing quite a bit of money until now, they've been doing very well. The whole industry has been doing well, but outsourcing has worked well for Frontier," he said.

"I don't want to speak on behalf of other airlines, but there's a lot of airlines, even out at DIA that use other third-party vendors. In fact, Simplicity works for another carrier out at DIA," Faulkner said.

Frontier is asking their customers to continue being patient.

"We definitely want to thank passengers for their patience and also want to thank all of our employees who have really pitched in and done a good job during this time of transition," he said.