It’s not limited to Reeder, it happens in the Newsblur web interface too.
I was looking at my list thinking it has been a couple days, so I switched my view to All, and refreshed for good measure, but no new articles.
Then I clicked on it, and two articles showed up.
One is still on the backing website, but the other is gone, so it did query but the new items aren’t immediately visible. I also checked Reeder, the items are not there at all until the web UI loads them.