Recently I found this question on LinkedIn:
It really depends on the what stage your team is in.
In the chaotic stage, you don't have time to wait around for people to realize that some team members are not taking responsibility. Talk to each of them personally and see if there is a good reason for them not to take that task . Either bend the process so that all team members are now able to take tasks, or get those out of order back into the team guidelines, until the chaotic stage is over. In chaos you can't afford to have people not on the same track as the others.
In the learning stage, ask the people what they are going to do about not being part of the task taking process. ask them to fix it themselves, and make sure they commit to what they are promising. follow up weekly or even daily. "grow" them to this habit, or "grow" yourself to realize maybe that habit isn't right for your current team. Sounds weird, but sometimes you need to remember to fit the process to the team, not the other way around.
In the self organizing stage, create constraints or situations for the team that will force them to face the choice of taking tasks on their own or not. For example, you can give requirements that no team member has specific specialization on, or the opposite. see what happens. you an also introduce people from other teams into the team, who might have a problem with this behavior and will speak up to the offending team mates on their own. The point is, at the self leading stage, you don't tell people how to do something. you just give them the boundaries in which to achieve it.