Below is a template that you can customise of a survey that you can use when members leave members. I recommend you use a telephone interview.
“Obviously we are disappointed with the news that you no longer wish to remain a member. But we understand that you had to make a decision and wish you every success. If I we can be of any further help then please let us know.
I do have one favour to ask. We like to learn more about what we could be doing differently. We have a standard set of debriefing questions that we like to ask when a member decides not to renew. Could I schedule a (5 minute?) conversation/meeting with you to go over them? It would be of great help.”
If they agree, you could send them the questions to reflect on before you call back.