Nobody owns it, so everybody assumes somebody does.

You have said some version of this out loud. The same few people do everything. The team was supposed to handle it. Nobody owns it. What you are describing is almost never laziness. Most of them work hard, and the ones quietly carrying the extra weight are usually your best.

Accountability thins out inside a group. One name against one number tends to get done. The same number handed to a team splits into fractions nobody agreed to, and no single person ever decides to carry less. It happens in the gap between a discussion and a decision, and everyone leaves the room assuming the owner was somebody else in it.

The cost is not the task that got dropped. It is what your strongest performers learn. They cover, because covering is faster than raising it, and it works right up until it does not. You get a small group doing the visible work of a much larger one, while the people beside them are never told they are falling short.

You will say your people already know what is expected of them. Some do. Ask each of them to write down the one number they are judged on and the seat they sit in, then put their answers beside yours. If the thought of that exercise makes you wince, you have the result.