Flows can only be mirrored once in any individual S Series switch. If you are installing Purview on an S-Series switch that already has mirrors configured be aware that if a flow has already been duplicated through a mirror it will not be duplicated again on a separate mirror. If the flow you're trying to mirror to Purview has already been mirrored some place else then it will not be mirrored again to Purview.
Also note N-Packet policy mirror (mirror the first 15 packets of a flow) is only ingress capable.
To export bidirectional netflow data, more than one port on the switch needs policy mirror applied.