Revvl 7 Swipe to Answer (sometimes)

I just answered a call which appeared with a button to answer. A few minutes earlier the phone rang, no button and only the option to swipe upward. When I tried that I was unable to answer the phone. The call rang out and went to voicemail. This is not uncommon. All I want is for one of them to work consistently but ideally both should work shouldn't they? Is this fixable and how?