Deluge
OK, this happens a lot:
Torrent that is driven by DHT alone, no trackers. A popular torrent with lots and lots of peers.
It downloads very quickly and seed for a bit before stagnating. No peers can be found. It is not that there are no peers in existence, these are popular torrents.
(If you want to emulate such a torrent and repeat my experiment, get a regular but non private torrent, add it in paused state and then delete the trackers before starting it for the first time).
What I think causes it:
The file torrents.fastresume contains thousands of IP address for the particular torrent. I don't think deluge ever contacted them all, but received their address via DHT.
After downloading the contents, it continues to look through this list for peers to seed to, only, these address are old as the torrent is small. It obviously takes far too long
to check all these address and deluge appears to be unable to seed the torrent. Even for days.
Shoddy work around:
I move the file torrents.fastresume to somewhere else (other than the folder ~/.config/deluge/state/) and start deluge again. All the statistics are gone, but the torrent starts to seed within 5-10 minutes at full bandwidth!
I have tried editing the torrents.fastresume file with a hex editor to delete the huge list of IP address, but I am unable to get the edited file to work with deluge (it is treated as absent).
Possible permanent solution to problem:
Look for new peers all the time, not just at the beginning (or when the list is dry).
Prioritise new peers in the search, as opposed to a FIFO list of peer addresses.