Please make sure that your target is not using a Bluetooth device or a Hands-free device to answer the calls. Also check if the phone is connected to the car system (Aux Cable) when the calls were made.
The application records the calls directly through the Mic of the target device. So the Mic of the phone has to be in use for the call, for this feature to work properly.
For more information you can visit our call recording feature page.