A new software problem has been found with Boeing’s 737 Max planes, which will likely delay efforts to get the jets that have been grounded since earlier this year after two fatal accidents back in the air. Boeing said the Federal Aviation Administration had identified the issue after it showed up in a flight simulator, also related the initial problem of software pushing the nose of the plane down automatically.

Boeing has been working for eights months to fix the software issues, since shortly after the first crash last October, of a 737 Max flown by Indonesia’s Lion Air. In both that crash and the second of a Ethiopian Airlines 737 Max in March, the plane’s software repeatedly pushed the nose down based on faulty readings from a lone sensor. There were a total of 346 people killed on both flights.