But buyer Indigo Partners says it's "hopeful" of resolving it in the next few days and closing the deal.
The private equity firm had set a Thursday deadline for agreements with Frontier's flight attendant union and a group tied to its former pilots union. Only the pilot group has made a deal.
Indigo said Friday that it's negotiating with flight attendants. It's also trying to agree with Barclays, which issues the credit card affiliated with Frontier, over extending the card and a credit line.
Phoenix-based Indigo has until Monday to tell Frontier's current owner, Indianapolis-based Republic Airways Holdings Inc., whether it wants to terminate the deal.