@mtele31784-0 Update: I redownloaded the torrent file for many of my previous downloads, and they are now seeding, but some of them didn't register that I have already downloaded them once before.

That means that the previously downloaded files are in the wrong place for your client to see them (or you've deleted them.) As long as they are in your client's download folder (or in another folder the client checks) it will re-check the files and only download missing pieces.