I recently bought a MacBook Air and I am not being able to capture a signature in a white sheet of paper with camera, as it was possible in previous machines and mac OS versions. Does someone have a hint on how to solve this?
As other have mentioned in the comments, it really seems to need a lot of light on the paper (probably so it can see the contrast). Much more than you would normally need for doing FaceTime call for example.
Place a strong light source behind your MacBook/webcam (e.g. room light, window, or flashlight) so that its light falls strongly on the paper with the signature. Then slowly move the paper towards the camera.