Did you disable uploading? If yes, try turning it on. I had the same problem, because I disabled uploading, my download speed went down.
And also how many seeds & leechers are there in the torrent tracker you're downloading from. These factors & the others mentioned by the guys who posted before I did, affects your download speed altogether.
http://thethirdtry.blogspot.com/2007/10/to-all-you-torrent-haters.html An old article I wrote (in computer noob-speak for my friends) about improving your BitTorrent speeds. Enjoy.