I've experienced this issue on my car too. I'm new to A8 ownership, this is my first, which I've had for a few months now. The drivers door button was not responding at all and so I was contemplating total removal, etc. Then I discovered this thread, gave the WD40 thing a go and I have since had intermittent function
Then, this morning, a below zero frost meant that the button was frozen solid, they all were
.
Recently I've been thinking about more WD40, to be then followed through with compressed air to blow out any residual debris, from the switch assembly.
I'm also wondering if the switch/handle assembly may have a drain hole, which over time, may have become blocked. I'll look underneath, next time I'm looking into the button issue.