Folder Overlayed cover or Full Cover not showing

Folder Overlayed cover or Full Cover not showing since new folders added.
I have my comic library on a NAS. I connect to it from a Mac as well as an iPhone. I noticed that the new folders I add-in don't show the beautiful overlayed cover or full cover. The folders I had when I first connected to the library still show the covers on the folder. Even without cover, these folders say "empty" when they clearly aren't.
In the browser, I'm still getting the cover on the folder.
Any idea what's going on?

iOS app version 3.17.1

Send me a screenshot please, and a screenshot of the settings section too.

I managed to fix it. I used the yacreaderlibrary on the mac and went to the folders that were not showing the cover and updated those folders specifically. This fixed the problem.

Ok, I would suggest a full library update to make sure everything is properly scanned.

I see. Maybe that's the reason why. In order to save time, I don't do a full library update. I have some main folders for each publisher. If there is no new comic for a publisher I don't update that folder. I just update folders individually. From now on I will do an entire library update each week.