I have a feed (ID 6067031) that only seems to fetch new stories about once every ~25 hours despite the statistics page saying it should be every 1.5 - 2 hours. From the log on the settings page:
2016-05-22 09:23:28 OK (200)
2016-05-21 07:52:06 OK (200)
2016-05-20 06:09:34 OK (200)
2016-05-19 04:52:50 OK (200)
2016-05-18 03:36:33 OK (200)
Additionally, the feed doesn’t seem to be storing any old/read stories. Some time (12h - 3 days?) after reading a story, it disappears unless saved. With the unread filter set to show all, I see only unreads, stories I at one point saved, and stories I somewhat recently read.
I’ve avoided doing any insta-fetches for a while, if that helps debugging.