5.1 on zte maven - slide down required to answer calls almost never works

When a call arrives, there is a slider icon on the phone. But it is almost always unresponsive to slide attempts. I have seen other comments on this but no definitive solution. I tried downloading an app that substitutes a button. It installed but never came up when the phone rings. I miss most calls because I can't get the slider to slide! Now and then it does, but seems random.