Why does the retention metric display N/A when filters are applied?
All the retention metrics are calculated on top of the first-time users in the application. If you apply multiple filters and the total count of returned users is less than 1% of the daily active users the retention metric will be displayed as N/A.
When this metric displays N/A for a particular day you can remove one or more of the filters. This way the returned users will be more than 1% of the daily active users and the retention metric should start displaying some numbers.