You behind a router?
Are you downloading old stuff?
If you are behind a router, you need to do some port forwarding or things will be SLLLLOOOWWW to non existent.
If you are downloading older stuff, check that are a lot of seeders(S) and not that many Leechers(L). If there arent many seeders(less than 15 or so) sometimes your DL will again be very slow or non existent especially since lots of people(seeders) limit how many people can be connected to them at one point as well as limit how fast they'll upload.