If you're experiencing issues with notification/mobile apps not receiving information/calls when expected it may be that the device has put the app to bed as it seemed unused at the time, meaning it might not be able to wake up on demand. You can prevent this by telling it not to go to sleep.
Open your Android device and visit;
Settings > Battery and Device Care > Battery > Background Usage limits > Never Autosleep Apps > select app
Once you've selected the app this should prevent it from being put to sleep in the background.
