Many cases of abuse and violence are not reported include blackmailing and because some people are embarrassed to come out to people.
If someone is blackmailing the person they are abusing with, lets say, their mother`s safety, they are not going to report it. They might think that if they tell the authorities and they go to the abuser`s home he will find a way to get out of trouble and hurt her or her mother worse than before.
Other people could feel embarrassed to admit to witnessing violence. Lets say someone was violating a kids belongings. That kid might be thinking "i cant tell my friends this happened because they will think I cant stick up for myself" and they never tell.
(These are just examples)
