Ok so I just bought some new songs from itunes and its the first time i've done that. I thought that if the songs that were already on my ipod were checked then it wouldn't delete them off of there. So I unchecked them and it synced the new songs on there but then the old songs were gone. When I tried to check them again to re-put them on there, every time I do the song listed on the list of songs just goes away... what does this mean!?