There are times when iTunes will stop importing automatically. I find it's when LW is too busy & particularly if it's been running for a long time & is particularly busy. A restart of LW usually does the trick. But phillipe99's advice is a little more complete. Are you aware that after installing new programs or system updates that you should Repair permissions after doing a SAFE booting (restarting whilst holding down SHIFT key until you see the message Safe rebooting.)

Oh I didn't read your post properly. You can't drag & drop into iTunes? Do you drag & drop it onto the icon in the dock or onto the itunes interface or program icon or onto the library icon above the playlists? There's a small difference between them. ie: iTunes behaves differently.

iTunes will NOT import an identical song file. But for itunes to refuse, the song file must be identical via both filename & id3 tag. So check your iTunes library to see whether you do in fact actually already have those songs.