Elseif doesn't work properly

I'm trying to make form validation with ajax. Mine problem is elseif statement doesn't work properly

    if (isset($_POST['submit'])){

    //Create safe values for input into database
    $username = mysqli_real_escape_string($mysqli, $_POST['username']);
    $password = mysqli_real_escape_string($mysqli, $_POST['password']);
    $company = mysqli_real_escape_string($mysqli, $_POST['company']);
    $mail = mysqli_real_escape_string($mysqli, $_POST['mail']);
    $adress = mysqli_real_escape_string($mysqli, $_POST['adress']);

    $errorEmpty = false;
    $errorTaken = false;
    $errorMail = false;

    //Check are required fields empty
    if(empty($username) || empty($password) || empty($company) || empty($mail) || empty($adress)){
        echo "<span class='form-error'>Fill in all fields!</span>";
        $errorEmpty = true;
    }

    //Check is username already taken HERE IS THE PROBLEM!
    elseif(isset($_POST['username'])){

        $username = mysqli_real_escape_string($mysqli, $_POST['username']);

        if(!empty($username)){

            $username_query = mysqli_query($mysqli, "SELECT * FROM users WHERE user_username = '$username'");
            $count=mysqli_num_rows($username_query);

            if($count!=0)
            {
              echo"<span class='form-error'>Username is already taken! </span>";
              $errorTaken = true;
            }
    }

}
    //Check is mail adress correct
    elseif(!filter_var($mail, FILTER_VALIDATE_EMAIL)) {
        echo "<span class='form-error'> Write a valid e-mail adress!</span>";
        $errorMail = true;
    }
    else{
            //Create user
            $add_user_sql = "INSERT INTO users (user_username, user_password, user_company, user_mail, user_adress) 
                            VALUES ('".$username ."', '".$password."', '".$company."', '".$mail."', '".$adress."')";

            $add_user_res = mysqli_query($mysqli, $add_user_sql)
                or die(mysqli_error($mysqli));

            //Close connection to MySQL
            mysqli_close($mysqli);

            echo "<span class='form-success'>Succesifuly registrated!</span>";
        }
}

Problem is when i check is username already in database,because after that step every next elseif statement won't execute. When I put it on first place it stops execution of rest of code,same situation is when I put it on last place.Other conditions for checking empty fields and mail validation are working properly.