I just saw this problem last night. I uploaded a torrent to a site and immediately had a leecher. The file was cruising along and after maybe 80mb up the leecher just dropped off.
I checked later and it said I had 5 leechers, but nothing was uploading. I just happened to try "updating the tracker" and immediately when I did that it showed I had 5 leechers and my upload speed was max out like it should be. But after about 10 seconds all 5 of the leechers fell of and my upload dropped back to 0, and it showed 0 people downloading.
I updated the tracker several times with the same exact behavior. Everytime I would update it it would connect to those leechers, but after a few seconds it was like the connection was lost or something.
Then this morning I am experienceing the same behavior with a different torrent at a different site. If I dont update the tracker every few seconds no one can d'l it.
I dont upload alot of torrents, meaning new stuff that I add myself. But I have down enough to know something is wrong. Ive never seen this before on any that Ive uploaded.