wp-rss-aggregator Categories not working

yesterday I bought and installed the Categories add-on for the wp-rss-aggregator plug in, to allow me to set up pages of feeds on various topics, but when I use the shortcode:
‘[wp-rss-aggregator category=’Humanism’]’
it displays on the page: No feed items found., despite there being three sources categorised as Humanism

If I leave the category out the feeds come through fine, and the ‘Excerpts and Thumbnails’ add-on that I added at the same time seems to work fine.

I have double checked that the license key is valid and activated but am still getting no feeds when Categories are selected

https://oxfordhumanists.org.uk/wordpress

I am using theme Twenty Fourteen

Thanks
Michele