In a weak matrix environment, two key team members are falling behind with their tasks,...

According to the PMBOK Guide, 6th edition, section 9.4.2.3, the project manager should use conflict management techniques to resolve conflicts among team members and stakeholders.3

Some of the conflict management techniques are: problem solving, compromising, smoothing, forcing, and withdrawing.3

In this scenario, the project manager has tried problem solving (holding a meeting to work out a permanent solution) and compromising (asking them to find a way to resolve the issues), but these techniques were unsuccessful.4

The project manager should not use smoothing (emphasizing areas of agreement and avoiding areas of disagreement), forcing (imposing a solution), or withdrawing (avoiding or postponing the conflict), as these techniques may not address the root cause of the conflict or may worsen the situation.4

Therefore, the project manager should escalate the issue to a higher authority, such as the functional manager, the sponsor, or the steering committee, who have more power and influence in a weak matrix environment.4

Escalating the issue to a higher authority may help to resolve the conflict, protect the project objectives, and maintain the team performance and morale.4

The references for this answer are:

1: PMP Exam Questions | 100% Free PMP Example Questions 2: PMP Practice Exam 1 | Free PMP Exam Questions 3: PMBOK Guide, 6th edition, Project Management Institute, 2017, pp. 349-350. 4: [Conflict Management Techniques for the PMP Exam - PM Study Circle] : [PMP Exam Prep: Project Human Resource Management - Simplilearn]