I know for a fact that a feed contains 50 stories. But when entered into NewsBlur (Premium) it only shows the most recent 10. Days later, NewsBlur continues to show only the most recent 10. Is this a bug or an issue with certain feeds?
Is it a Reddit feed, or something else? I haven’t noticed this with any other feeds, but I have a few subreddits that I follow for pictures (/r/EarthPorn) and what you describe is what I have been seeing. But strangely, some other subreddit feeds are working fine.
I cannot pin it down to a specific website or service though it’s often with feeds from RSS.app because we use that service so much. Sam once told me that the NewsBlur algorithm will not download stories if it thinks you’re not reading the feed. So I marked a few articles as Read. This worked with one of the feeds but not the others. 
NewsBlur will download new stories, even if you aren’t reading the feed. It just won’t keep more than a few until you start reading them again. It’s a decent tradeoff and you can pay for the Premium Archive tier which keeps all of your stories.
We mark every feed as unread each week yet some will not show more than 10 articles. We don’t need every article ever published but we need every article published during the past week or so and some feeds don’t do that even though we read them and even save articles from them.