Shortcode to show random featured

Hi there,

i use Geodirectory now some month and now i have noticed that the shortcode gd_listings i use to show ramdom featured listings on my website shows always the same listings. Nothing changes not by reload of the page not by using other browser for the page.

Should random not suggest that there will be listings shown different each time page loads?

This is the Code i use:
[gd_listings post_type="gd_place" post_limit="2" add_location_filter="1" show_featured_only="1" sort_by="random" title_tag="h3" layout="2" view_all_link="1" bottom_pagination="1"]

Any ideas wht i need to change so that with a realod of the page a different listing will be shown? Or do i nee to use a different shortcode?