Starting with Android 6.x, some phone vendors have tuned Android to hibernate apps that don’t appear to be in use by the phone’s user. This would include background services such as SMS Tracker. To learn more about managing your phone’s power settings click here
Also, make sure you are running SMS Tracker version 4.004 or greater. This release does a better job of reducing the power consumption of our background service. By turning off GPS polling, SMS Tracker is significantly less likely to be stopped by the system when its looking to conserve battery life.