Hmm. The inactive feed explanation doesn’t match my observations.
The feeds seeing exactly 20 items coming in at a time are active on a daily basis. And I read them regularly.
I wonder if something like a combination of increased tweet volume and/or long-ish polling intervals has revealed a limit that was always there.
It looks to me like the twitter API that tweepy uses to fetch tweets has an optional “count” parameter that (as far as I can see) is not set to any particular value. I also don’t see mention of any default limit in the twitter API docs, but clearly there must be some limit to how many “recent” tweets are returned. I wish I had a better understanding of the interplay between polling intervals and the definition of “recent tweets” in the twitter API.