The solution to your problems comes in the form of in-app messages. Apps that use them have a retention rate of 46% after three months of being downloaded. Compare this to the 13% retention rate of apps that don't use them. They sound great, but what are they?
In-app messages are, just as their name indicates, messages that appear while someone is using an app. In-app messages differ from push notifications in one crucial way: the former only appear when the app is being used, while the latter pop up when the app is not running. Another great and unique thing about in-app messages is that users can't opt-out of seeing them, which means that they can reach 100% of your subscribers.
While just having in-app messages helps you keep your audience, using them in the right circumstances can boost their efficiency. For instance, it is better if users receive such a message after using the app in a specific way, rather than receiving it at random.