Your share ratio is 0.001, you big leech.
Only joking. it did it to me once. I cant remember how I resolved it exactly...
But I did the following
Checked Firewall
Checked port forwarding on router
Changed TCP listening port.
I think the last change solved it..
Oh, and make sure your not uploading too many torrents at once, as this can prevent downloads.
Or, like others have said, use a different client. Personally I dont mind Azureus. I usually get around 60-120Kbs download speed. But then, I tend to upload at 50kbs