If you're logged into last.fm, you're stuck with their settings, but if not, you can configure this in.preferences. now playing preferences, I believe, but it might be player preferences.
Edit: I see you mentioned that in the first post, sorry. It may be that you're not seeing the play count update until the end because mb waits to release the file before updating it. I suggest experimenting with stopping the track part way through to see if the play count is incremented after stopping.