Recently (August 7), Booster 9 conducted a static fire test where it only ignited 29 of 33 engines. This is the big static fire, there not another one. I have found that they were shutdown prematurely a lot (example), but no where does it say why. So, what happened, and why were 4 engines shutdown prematurely?
SpaceX has not released that information at this time, and may never do so. They have no strong reason to publicize it, really.
The engines will shut down automatically if they detect any of a wide variety of issues, so each engine shutdown could have been anything from combustion instability to a bad sensor that didn't report correctly under live conditions. A premature engine shutdown is a bit like a car's "Check Engine" light; it doesn't really tell us anything except "something went wrong".
I think it's more interesting that the test was meant to be 5 seconds but actually ended just short of 3 seconds. That suggests something happened halfway into the test that caused a full system halt.