If, when it finishes downloading, it moves into the completed decion and shows 100% and "seeding", it's seeding.
Don't believe the number of seeds and peers it shows.
Also check at the bottom of the Utorrent screen.
Does it show a yellow triangle with a ! in it?
If it does, it's not getting incoming connections.
It should turn to a green circle with a tick in it.
Double click on it for a speed and port test window to open and let us know if it's got a problem.
As Pyro says, you can only seed if someone is trying to download it. I have loads of torrents with no seeds or peers (and do the recheck just in case).
Finally, try stoppin it and the starting it again (right click file).
Do let us know.