https://arstechnica.com/information-technology/2020/01/737-max-fix-slips-to-summer-and-thats-just-one-of-boeings-problems/

The past 10 months have not been good for Boeing for all sorts of reasons—capped off by the failure of the company's Starliner commercial crew vehicle to achieve the right orbit in its uncrewed premier in December. But the biggest of the company's problems remains the 737 Max, grounded since last spring after two crashes that killed 346 people between them. Combined, the crashes are the worst air disaster since September 11, 2001.

Both were at least partially caused by a sensor failure with no redundancy and a problem with MCAS (the new software controlling the handling of the aircraft) that the air crews had not been trained to overcome.