Can also be caused by client session closing incorrectly (like when your pc shuts down abruptly) so it doesn't report correctly to the tracker. Solution is either to wait until the tracker figures it out, or restart/reshut the client. Enabling graceful shutdown in uTorrent advanced settings (set bt.graceful_shutdown to true) helps too, especially if you have a lot of torrents. Don't know how the last is applied to vuze however...