Improving retention

Hi All - this is my first post!

Does anyone have a good approach to root cause retention challenges?

The reviews for my mobile apps are decent, but less than 3% of users remain active after five weeks. I have mind mapping apps that get decent reviews, but they are missing cloud syncing and desktop experiences. I feel like not having these features is causing churn and limiting user-value. However, developing them will take a while and be costly.

Is there a better, low-hanging-fruit approach finding the root cause of retention problems?