Oh! What if the wards only work inside the castle? Hermione's battle with the troll was in the sunlight and thus outside. While I would hope that the wards would cover the entire school grounds (rescuing students from the Forbidden Forest seems like a more common use case than rescuing them inside), that's another possibility that seems somewhat more reasonable than some of the others I've listed. (I should also, for completeness, add the "he responded to the injury alarm but it took too long" option, but I find that one unsatisfying for several reasons.)