Nevermind - figured this out after browsing through the forums more.
For anyone with this problem in the future - you can only have a certain number of torrents actively downloading at any given time.
If you've downloaded selected parts of a torrent only and are seeding it (IE, you've technically not got the 'complete' torrent even if you have got the selected parts you wanted), it will count toward your limit of 'active torrents' - even if your client displays it as complete.
To fix this, either download the torrent in full (after which it won't count toward your limit), or stop seeding the torrent.
The explanation is here:
https://forum.gaytorrent.ru/index.php?topic=63588.0