audio but no video with apple tv on samsung tv

Some content may not support AirPlay, and then might stream only the audio part.

Some content may require a certain minimum model AirPlay-receiving device, and then might stream only the audio part to older models.

What type of content were you trying to stream?

What is your model Apple TV? Identify your Apple TV model - Apple Support

See the AirPlay support on the sites of various content providers.