Screaming atiel, how to stop?

I have a atiel and three other birds. They all get along and talk to each other. I have them in a place where they are surrounded by people a lot of the time but not all the time. The atiel is a screamer. I have hard that it is because he wants more attention but I have always given him a lot of attention. I have also heard people say it is because if they do not have a mate they look at their masters in that way and scream when they cannot see them. He is getting worse and I just don't know how to get him to stop any thoughts?