Some Torrent users don't re-seed a file on request because they don't know how to do it.
Just click on a torrent link (or open a saved .torrent file) and select a filename of a file that has already completed. BitTorrent will check over the file and realize that it's already complete, and continue to connect to the tracker and serve as a seed.
Please remember that sharing only works when all useres help to share. If you have problems, look up this useful website
http://dessent.net/btfaq/#seeding
YourValentine