Amending song information in iTunes. Not syncing to iPhone

I have CD albums manually loaded onto iTunes for years and have had them synced to my iPhone. Now I want to amend the song title and artist detail. I'm trying to tidy up all my manual songs with correct song titles and artist information instead of simply having "Track 1" etc. I right click on the song and amend in "Get Info". All ok in iTunes, until I sync. After syncing, the song details remain unchanged on my iPhone. Why won't my iPhone sync up properly with the amended song information I provided in iTunes? Help?