It's difficult to say without more details or without seeing it directly, but generally you can check that the tracker is up and running, those torrents have any active seeds uploading, and your connection is working properly and the required network ports are forwarded to your computer.
I may suggest trying out uTorrent too, it's these days the most popular client around and quite stable and recommended too. You can resume the half downloads from the other client if, after adding the torrent, you copy the partial downloads to the destination folder and then start the torrent, it should recognize and start seeding what you have and downloading what you don't.