I have similar issues. I figured out that subcats will not shown automatically, but if you do it like that
[showcase source="category: 7,11,13,14,15" limit="6" show_more="yes" show_more_item="3" loading_animation="sequentially" filter_deeplink="yes" page_deeplink="yes" filter_style="2" popup_category="yes" popup_date="no" popup_detail_button="no" medium="2" large="3"]
it will show the cats, but no article, how can I solve this?

Hello,
please open a new thread, your problem is not related to this problem.
Live Url please or Screenshot