Hi, if I understood correctly you have two issues:
1. you are having difficulty in structuring the problem in a way that it is MECE
2. you don't know much about customer retention
About the first point, you are probably not breaking down the problem correctly, or you are mixing different factors (try to keep breaking them down until you isolate all the components).
About the second point, try to use the common frameworks (4Ps, Porter, etc.). If they are not useful (I think they should be) then try to read some literature about customer retention. I personally think that instead of learning frameworks it is better to develop some business sense