Camera covered

Hello, I have an iPad Pro 5th gen. When I try to use Face ID I get the message camera covered when it isn’t. Neither is the depth finder and all the other possibilities mentioned on various sites. I’ve also tried to hard reboot suggested by this forum a couple of years ago but that doesn’t help either. My forward is up to date 15.6.1

does anyone have any idea how to solve this? Many thanks in advance, Philippe