[Garima Sahore] As a scrum master, I make sure to have the retrospective invitees sent couple of days ago to the team which helps them in planning the other activities.
Couple of things to consider:
1) If its just one instance, Its understandable. There can be something which is at a program level and PO has to be part of it.
2) If the PO always gives the reasoning that he is super busy then it needs to be retrospect one to one. PO should be coached on why we conduct retrospectives as it is the only ceremony wherein the team inspects, adapt and improve the way of working.