How come my own seed shows as 0% availability?

I usally leave everthing I download for twice the time it took to be fully downloaded, thus leting people have it. Nevertheless sometimes it shows: Availability 0%, how come if I leave it there with all the task proerties selected to enable the upload with the best of my (not so fast) Internet band. Am I doing something wrong?
I apreciate any sugestions.
Thank You.