As a PM, In functional and weak matrix organization, a staff working for your project not performing well what action you will take? (this staff is reporting to another manager)

the manager in charge should request for an urgent briefing with staff to acknowledge them about the critic situation and listening to their explanations, taking in account if there is not serious issues preventing the smooth work, so you may come up with solutions ending the crisis.

If it's not the case, alerting the staff about the unacceptable behaviour and a series of disciplinary sanctions will take place in order to get back quickly performance recovery .

a daily briefing is also recommended with the manager under the charge of the project manager or establishing a quality unit to insure that the quality standards of work matches the expectations.