I have an LG K7. Think it's the Android Lollipop 5.1 .Rarely it would answer, sometimes not. I tapped and swiped the icon repeatedly.
Never read the instructions. Turns out I have to place my finger on the ringing icon and keep my finger on the screen as I move the icon to the right, or probably anywhere on the screen. Just tried it four times, worked every time. So, unknowingly, sometimes I was doing that and sometimes not. That's why sometimes it would pick up and sometimes not.
Hope this helps.