It's tough. From my experience, when you make a general announcement then the one who needs to hear it thinks it is about others and not her so she ignores it, and the ones who do not need it think it is about them and overcorrect. An example would be discussing tardiness. Those who are tardy will not think you are talking to them and continue to be tardy, and those who are on time will arrive even earlier. Same with talking...those who talk will ignore the comment and those who barely talk will clam up. It's hard to talk to the specific individual, but sometimes it's the best way (or only way to get results).