The world's largest aerospace company is now facing multiple lawsuits in both crashes, including one that alleges Boeing concealed problems and refused to ground the plane on its own. But Boeing Co is facing increasing hurdles in obtaining approval to return the plane to service before the end of this year as it has targeted. It cited "continued uncertainty around the timing of MAX returns to service" by the FAA, CNN reported.
United Airlines, the other U.S. 737 MAX operator, had thus far cancelled flights into January, although it may yet have to extend that time frame.
Earlier this week, regulators found gaps and substandard documents in the recertification paperwork submitted by Boeing, and asked for revisions.
One of the issues raised by FAA regulators is that Boeing presented the MCAS documentation in a similar format as it had been in the past, but officials wanted it in a different form.
The delays have thrown into question when Boeing can complete a certification test flight.
The 737 MAX is grounded by aviation authorities worldwide after two of those aircraft crashed in a short time. Boeing has said it expects to receive regulatory approval to resume flights in the fourth quarter of 2019.
Southwest had previously extended the grounding of its 34 737 Max jets to February 8, the latest of any United States airline.
Without the planes, Southwest says it will cancel about 175 flights each weekday.
That led to an extensive redesign of the plane's flight computers that has dragged on for months and repeatedly pushed back the date of its return to service, according to people briefed on the work. While Southwest has always had an all-Boeing fleet and has tied its future growth to the Max, the airline recently said it would review whether to add other aircraft types. The airline is discussing compensation with Boeing, but no agreement has been reached.