Answer by TheDeadMedic for Ajax Load more CPT via shortcode

This will (*should) fix your first problem:

function rmcc_post_listing_shortcode1( $atts ) {
    // No need for output buffering - AJAX handlers should ECHO their response
    // ob_start();

    // ..

    // So long, farewell
    // $myvariable = ob_get_clean();
    // return $myvariable;

    // Now terminate
    exit;
}

Your second problem is that you are not implementing any kind of pagination/increment in your AJAX handler - "load more" will always load the same post.