Officials investigating why a panel on a Boeing 737 Max 9 blew open during an Alaska Airlines flight last week say they are struggling to piece together exactly what happened because the plane’s cockpit voice recorder overwrote itself before it could be retrieved.

This is not a new problem. The National Transportation Safety Board, which is leading the investigation, has recommended for years that recorders be programmed to capture up to 25 hours of audio before automatically resetting themselves, but the Federal Aviation Administration has been reluctant to mandate longer recordings.

The F.A.A. last month proposed 25-hour recorders on new planes but argued that adding them to the existing fleet of U.S. planes would be too expensive. In addition, a pilots’ union has opposed the move to 25-hour recordings unless Congress puts in place protections that would prohibit their release to the public.

The chairwoman of the safety board, Jennifer Homendy, said the agency’s investigators had conducted 10 investigations since 2018 in which the cockpit voice recorder had been written over, with critical recordings lost forever. The voice recorders are among the key pieces of evidence that investigators use in reconstructing the events that led up to accidents as they work to establish a cause.