If you are using an Android device and the phone is running in Low Power Mode or a specific Battery Optimisation Mode, for the phone to conserve battery energy (and not completely wear it out), the app shuts the app off from listening in the background as it would utilise too much energy for the phone to handle. This is a service that is built in to the device to protect the longevity of your battery (and device). 

In order to ensure that our apps continue to play in the background or while in idle, please switch off any battery optimisation mode that may be running on your device.