Some places are "two party consent", both parties have to be notified that the call is being recorded (which is why you sometimes hear that "your call may be recorded for ..." thing - it is being recorded). So most manufacturers find different ways to prevent an app from getting the incoming voice.
Call Recorder has a number of "work-arounds", and one of them usually works, but you have to try each one until you find one that does for your phone.