Iphone music doesn"t show up in itunes

I just upgraded my iphone 3G to 4.0. It is jailbroken to unlock it as it is an AT&T phone and i live in taiwan. I am syncing it with itunes, and can copy music to the phone, and it plays it, but for some reason the music on the phone does not show up on my computer. Does anyone have any idea why this would be? and or how to fix it. any help would be great.
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I have tried many different things. i can drag and drop music into my phone and it is there on my phone to use, but my computer still does not see it. my podcasts show up as well do some playlists. but the rest of the music is empty. this makes it quite difficult to add or remove songs, seeing it syncs ALL songs every time. does no one have any idea? Please help.