In order for a follow.it-feed to be filtered by tags or authors, the underlying RSS feed needs to tag every message with tags or authors.

Not all publishers have set up their RSS feeds this way. In such a case, you can reach out to the publishers and suggest them to do this.

Alternatively, you can use the keyword-filter which doesn’t require any tagging by the publisher. If you add enough keywords then you will probably not miss out on the key stories, as they should contain relevant keywords. 

Note: you can simulate which keywords to pick before following a feed.