Ratings are only available from the app stores as a daily cumulative snapshot. We only start fetching ratings when they are being followed by an active Appbot account. This means that we are unable to fetch the full ratings history. If the app has previously been followed in Appbot we may have some historical ratings, but there may also be missing periods.
For Google Play ratings, if you can connect your Google Play Console to Appbot then we can access some historical ratings. Unfortunately, Apple does not make the historical ratings available at all