Email newsletters don't update unless NewsBlur open in-browser

I’m having issues getting updates from email newsletters via NewsBlur and the API to a 3rd-party reader (NetNewsWire).

It seems that the NewsBlur-generated feeds for newsletters aren’t updated unless NewsBlur is currently open in-browser. Whenever I don’t have NewsBlur open, repeated refreshes in the 3rd-party reader don’t show new items in the newsletter feeds, but opening/refreshing NewsBlur in-browser immediately shows updates to the newsletter feeds, and a refresh via the 3rd-party app just after that also shows the updated items.