There are a lot of factors that contribute to a kid going off the Derech that we don’t understand especially when judging from the outside.
In one case I know of, one child in the family was born with a uniquely challenging personality. He was never diagnosed as ADD or anything strong enough to warrant medication.
However, as the schools years went on, he went in a wrong direction. His difficult personality contributed to his lack of connection with parents and other positive adult mentors.
Each personality of every child in a family is different. Although the parents may be good parents, for reasons beyond our understanding, they so far have not been successful with THIS child only.