I presume torrents download ok from your PC, so its not a port problem?
If so, within the P2P application. Click on settings on the left hand column. Make sure its actually enabled. Enable auto download too - that should start it.
To manually start them at the far left of each torrent file, after the percentage complete bar, and upload/download speed there should be a green arrow. Click that, should start.
If this doesn't get it started then i'd login via the config side of the NAS and do a factory reset. Seems to fix a few problems.