Like other USA 737 MAX airlines, American is scheduling without the aircraft until the FAA approves new software and training updates by Boeing after two fatal crashes.
In addition to the tragic loss of life, the crashes and subsequent grounding of the jets in March has cost the company billions, and Boeing is in the process of reaching settlements with families of the crash victims.
The world's largest plane manufacturer had submitted documentation in a vital part of an approval procedure, already postponed by months, for a 737 MAX software improve in the wake of two accidents in Indonesia and Ethiopia that killed 346 people.
The issue with Boeing's audit was brought up in recent days at a meeting in a Collins Aerospace Systems facility in Cedar Rapids, Iowa, attended by officials from the Federal Aviation Administration and the European Aviation Safety Agency, said the person.
EASA Executive Director Patrick Ky said after the Iowa meeting, "we think there is still some work to be done", according to Janet Northcote, a spokeswoman for the agency.
"Regulators have requested that the information be conveyed in a different form, and the documentation is being revised accordingly", according to the statement.
One source briefed on the matter characterized the problem differently and stated Boeing's documents had gaps, was substandard, and meant regulators couldn't complete the audit, a vital step before the airplane will be certified to return to service.
CEO Muilenburg was grilled at a US Congressional House Panel Hearing later last month over telling half-truths linked to potential glitches in its autopilot software of which Boeing Co. pilots had not been trained or notified either.
Muilenberg told a New York Times conference he thought about it, but decided it wasn't in his character. The company will then fly a certification flight as part of the FAA's review of its changes. Southwest Airlines is being more conservative, keeping the Max out of its schedule until February 8.
Boeing has repeatedly delayed its timeline for returning the plane to flight by the end of 2019, as regulators have scrutinized proposed fixes.