Unable to delete members

I've made a page where I want the site admin to be able to delete users. There is a textbox for the username and a button to delete that user. The code behind is this.

   Protected Sub btnDeleteUser_Click(sender As Object, e As EventArgs) Handles btnDeleteUser.Click

        'delete user and get result
        lblRizult.Text = Membership.DeleteUser(txtUserName.ToString.Trim, True).ToString


    End Sub

It just returns "false" and looking inside the users table nothing has changed

What am I missing?