With your Pandora Premium subscription, you should be able to download as many stations as you want (as long as you have enough storage available on your device).
Is it possible that this might be occurring because there is not enough storage on your device?
Pandora will never use the last 500MB of storage, so we will only download as many stations as we can before we hit that limit.
If it doesn't appear to be storage related:
I'd recommend signing out of the app and signing back in, which will reset the download process.
Then, make sure to keep Pandora in the foreground of your device while the download completes.
If you continue to have trouble:
Check the download icon for the content you are attempting to download. The two rotating arrows indicate that the content is still in the download process and the highlighted downward pointing arrow means the content has successfully downloaded.
If the icon is not highlighted, try downloading the content again and keep the app in the foreground while your offline music downloads. Keep in mind that stations and large playlists take a longer time to download.
Lastly, if you are having trouble downloading stations, please try downloading a single track for offline listening to see if that works.