Make sure that your phone is connected to network.
Check the settings of your phone.
Use Oppo A77(Android 6.0) as example：
1. Go to Settings -- Battery -- Energy Saver -- select the Letstalk and turn off -- Freeze when in Background, Automatically Optimize when an Abnormally is Detected.
2.Go to Settings -- Notification and Status bar -- Notification Manager -- Letstalk -- Allow notifications.
3. Go to Phone Manager or Security Center or Security -- Privacy Permissions -- Startup Manager -- and turn on the button.
4. Click the menu button and slide down the application page until you can a lock icon in the upper corner to allow applications to run in background. Then the application will keep running in background.
5. Make sure that Low Power mode, Airplane mode and Quiet Time or No disturb mode are turned off.