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1. Get another member to complete the activity so that it does not have an impact on the milestones.
2. As a parallel activity, discuss with the member and if you are a PM with matrix reporting escalate the issue with the reporting manager for that team member. If you are the reporting manager for that team member, discuss and give indicators as forewarning either directly or through other team members. The PM can try assisting through his or her network channels to resolve the team members issues (if it is an issue).
3. If the team member continues on the slack frequently and not able to acknowledge the impact on the larger picture (that of the project) then it would be advisable to keep your supervisors informed and loop in the HR on performance management issues if needed.
Fact finding is the basic key. First you need to know the reasons then you can address them. This is possible by regular counselling sessions. The more you interact with the staff, the more you are able to get maximum out of them. Rotation or changing the staff should be the last option.