One of your team in a project does not perform well, what is the BEST action you do as a project manager?

Project Manager faces these kind of problems at project see if you can move that resource to another assignment at the same project or talk to him in the areas where he/she is lacking.

best thing is to understand the reason behind, then you can act accordingly

Interact with him performing the same task (by the end he need to be learned)

set reward system for both team work and distinguished indviduals 

Problem always has causes, understanding it makes most of the problem go away. Talk to the person.

• If the problem is skill, switch the person to another project or help him out.
• If the problem is understanding, better explain it well.
• If the problem is the team, pitch in and make him/her comfortable
• If the problem is the amount of task, break it up to smaller achievable milestones
• If the problem is the person and how ever you try to help it does not work, lose hem/her.

This happens to me sometimes eventhough I'm not a project manager but rather a SCRUM master, what I usually do is to talk to the person or team member/s and ask him/her on impediments/showstopper that his/her team is encountering then what can be done on it. But if the problem really proves on the competency, unmet expectations or lack of accountability, then I'll issue a verbal warning, followed by written one then last resort is the dismissal of the team member.

If it happens that you are the subject of such situation...always ask yourself, "Why am I on the payroll?". c",)