According to the PMBOK Guide, the project manager should collect requirements from all stakeholders, using various techniques such as interviews, focus groups, surveys, etc. The project manager should also document the stakeholder requirements, expectations, and assumptions in the requirements traceability matrix.
If some stakeholders are not available for a meeting, the project manager should use other communication methods to reach out to them and gather their input. This will ensure that the project requirements are complete, accurate, and aligned with the stakeholder needs and expectations. References: PMBOK Guide, 6th edition, pages 140-141, 152-153.