Doze is a feature that arrived with Android Marshmallow that improves Battery life by placing your phone in a deep sleep mode when it realizes your phone is sitting still and is not in use. The feature limits network access for apps. Doze will allow for SMS and phone calls to get through. You can actually enable other apps to get through the doze wall. This is great if you don't want to miss notifications from certain apps.
-Open settings and go to Battery/Power
-Choose Battery Optimization from options menu in the top right corner
-tap filter and select apps
-tap on the app you want to change settings for
-tap "Don't Optimize" then tap "Done"
via Phandroid