It's common for a car to be able to drive routinely over a soft spot, but get stuck if you stop, turn a wheel wrong, or more important, keep spinning when you start getting stuck.

The rover getting stuck might have been more about how it drove through the spot--the drive protocol--rather than something particular about the spot. A combination of a low spot, maybe one wheel sticking for a minute, and then spinning the wheels for a long time did the job.

Obviously, the rover is driving quite nicely over similar material now. Maybe a more careful drive protocol will do the job, but I don't know enough about it to know exactly how they would fix it. I don't think their equipment for detecting hazards is fine-tuned enough to be able to tell within seconds if wheels are turning but the rover isn't moving.