When everyone on a team knows about a problem, the likelihood that any individual employee is going to mention it to the boss goes down.
There are at least two reasons for this. First, if everyone on the team knows about it, employees may assume that the boss must know about it already. Second, when the entire group is aware of a problem, there’s a diffusion of responsibility–every individual has a sense that someone else will report the problem.
I/O at Work has a good summary here. The original article, The Voice Bystander Effect: How Information Redundancy Inhibits Employee Voice, appeared in the Academy of Management Journal.