A8Lowner

Hello, I have a 2005 A8L with the 4.2l v8. When I first start my car my park assist is on and works perfect but when I start driving past a certain speed it turns off which I think is normal? But then when I go to park again it is still off and I have to turn it on by pushing the button were as it used to just continue working before. I inspected the sensors and they seem fine. Does anyone know how to fix this? Does it mean one of my sensors is bad? Please help!