Hi, I believe you’re only looking at Unreads. If you click on the Unread Stories filter in the top right, you’ll be able to see All Stories, which goes back some number of stories. That number depends on if the feed has any premium subscribers and how often it publishes. Premium users get to enjoy a longer feed archive, but it bottoms out at 100 stories. So the stories are there, they are just unread.
I’m working on a pro tier of premium accounts that has unlimited unreads and an unlimited archive, as well as boosted feed fetch frequency. It’ll probably be $19/month, but that’s also not out of the range of an enthusiast reader.