Some HTC Android 6.0 phones have a feature named "Battery Optimization", you can control an app to allow / disallow it work in the background.
You should set Boldbeast Recorder as a "Don't Optimize" App, otherwise sometimes when a call comes the ROM doesn't notify the recorder, therefore some calls may not be recorded. By setting Boldbeast Recorder as a "Don't Optimize" App it works perfectly.
Please in the built-in "Settings > Power > Battery Optimization" screen to change it. Some phones may be in a different screen.