Start off by saying that the goal of the post-mortem is not too blame anyone or point fingers but to find out where improvements can be made and where downtime and incidents can be reduced. Every time a team is paged at our company we have a post-mortem meeting to review the incident. If we have a lot in a specific week we do them in batches when possible. We pull in anyone involved with identifying, triaging, or resolving the incident and create follow-up tasks based on the meeting. You might want to try the Five Whys approach as well: https://en.wikipedia.org/wiki/Five_whys