some times if you the tracks aren't named properly, last.fm can't scrobble them.
i use windows media player and if it's not scrobbling a track i have to go into the library and make sure that the artist and track names are in the appropriate field, instead of as one long title under track name.
hope this helps.
I still don't know what the problem was, could've been mislabeled tracks, but I can't think why since I'm pretty meticulous about track names. Hmmf.
(I eventually exited the last.fm program and reopened it, and that seemed to fix things.)