itunes and scrobbling

i've noticed that sometimes, my last.fm client will not scrobble songs i've played on my ipod properly. this seems to happen if i've played a large quantity of songs (more than 60)--none of them register. i am sure not to play any songs from my computer before doing so, so it's not a time sequencing issue. any one else experience this? it works fine with a small number of songs, but if i have my ipod away for several hours and am listening the whole time, the tunes are lost
thanks!