Sharing torrents in a private group is a zero sum game. Ideally you have everyone with a 1:1 ratio but if some users go far above that they are taking ratio from other users who might be fully willing to provide upload, but simply didn’t get there in time. You can set torrents to close after they reach a specific ratio, but that also means bringing down the availability of that torrent.
Also, it’s much harder to get to a 1:1 ratio if you only download less popular content because no one will download it from you for a long while.