Hi,
This could be a few different issue. I will offer 1 suggestion below for a potential fix (for android devices).
If that does not work (or if you are on IOS). Please email us at support@j-novel.club with screenshots of the series tab in the app.
It may just be an issue of needing to refresh the app.
To do so, delete and reinstall to clear the cache data.
If you are using an Android device, please also go into Settings > Apps > J-Novel Club > Storage > Clear data