Retaining your users it is hard. But if they stopped using your app recently, showing love and a message reminding him why he downloaded the app in the first place might be enough.
When you use Liquid's Formulas, you'll select the audience, the action and the time.
Inactive users on last 15 days
Send a Push Notification
Send it next day at the time the user usually used the app
Define a custom segment with users that have not used the app for more than 15 days.
Select Push-notification as the action and write a message that highlights what the user benefits by using the app.
Schedule the time for the next day and select the “Send at the optimal time” option.