Informal methods of social control do not work in our society because our society is too big and too heterogeneous. These methods of social control tended to work better when we lived in smaller towns that were more homogeneous.
Informal methods of social control are things that are done by one person or group to show that they approve or disapprove of the actions or behaviors of another person or group. These can be very subtle things, such as simply not smiling at a person who says something to you that you think is inappropriate. It can also be more direct. For example, an adult might tell a child that the child’s behavior is not appropriate for the situation. In either case, there is no official action being taken to sanction the offending person.
Such methods do not work well in our society because we have so few connections to other people. If high school students are walking down the street using bad language, they are not very likely to worry about people the approval or disapproval of people they meet. They do not know the people they meet and they can be reasonably sure that the people will not know them and their parents. Therefore, they do not really need to worry about getting in trouble. In addition, because our society is so heterogeneous, we do not all hold the same values anymore. Therefore, we will not all give the same signals to people. We might not all care if the high school students use bad language. Therefore, the students will not always receive the same message and their behavior is not very likely to be affected.
Informal methods of social control were much more effective (and still are) in small towns where everyone knew each other and most people held the same values. We are no longer such a society so those sorts of social control are not so useful anymore.