Whenever I make a duck sound. Charlie comes straight to me. But he doesn't come when I say "Charlie come"

Like if he's getting into some food my daughter just dropped from her chair. And I say Charlie come, he doesn't come. But if I make the duck sound he comes straight,and sits down no questions asked.

Why might this be?

I need to go find my belt. Last time I leashed him to my loop and the loop broke. lol