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 link
Depending on the client a person uses, (like on Burst!) you must select the main folder you saved the completed files to, not the filename or their folder. (ie. files are at e:\ dvdtorrent\somegig-somedate\, then to resume you must choose e:\dvdtorrent (the foldername somegig-somedate is in the torrent already). Simple...