I know this will sound condescending and I apologize in advance, but did you restart your computer? This has happened to me on occasion and a reboot has always fixed it up.
Ha- my 12 year old just showed me why stuff I buy from iTunes is shown in my library on my computer, but can’t be found on my actual iPod Touch.
It seems one needs to click on the item in question and drag it from library to pod, and poof, like magic, it appears in the pod!
Gah.
On the one hand, the iPod has kept me entertained enough with the things I can do with it since I got it last Christmas that I haven’t really missed the purchased items, which have just been hanging out in the library.
One the other hand, I might enjoy the thing more if I actually read the directions or watched the tutorials on how to use it.
There are settings to control when or how things are synced up. With my iPod, everything I download is automatically put on my iPod the next time I sync. But I have 30 GB to play with and haven’t filled it up yet. My wife’s iPod is set to only sync the stuff we tell it to because it can’t hold everything at once. Check the Preferences in iTunes if you want to control how yours works.