Last.fm scrobbling not working

Love the app so far - but having trouble with last.fm scrobbling.

I enter my username and password, and tick enabled, however when I check last.fm it isn't scrobbling. And when I return to this preference screen, the tick is gone. I retick it, leave that screen and return, and it's gone again! It is remembering my username/password though.

I'm on Snow Leopard.

Thanks!