Selective Scrobbling

This has been going on for years now, and it's always bothered me. I don't know why it's taken me so long to finally ask about it, but here's my issue. When I go into the Last.fm plug-in for iPod Scrobbling (Help > Diagnostics > iPod Scrobbling) and tell it to scrobble my iPod, it usually is able to get all the songs I have listened to off of my iPod, but on occasion, it won't find any music I have played or only a few tracks I have played. I would like to know what might be causing this and possible ways to fix it, if there are any.

Thanks in advance!