Why Can't I See Downloaded Songs on My iPhone?
When you manage your personal library of MP3 music files using iTunes, the software prepares the files to be synchronized to your iPhone the next time you connect it to the computer. Some of these sync, restore and backup sessions, however, may not transfer properly, leaving you to later notice that some of the songs are grayed out and missing or unplayable. The glitch can be cleared out by reconnecting and manually resyncing the iPhone with iTunes.
1 Sync Again
Reconnect your iPhone to your computer and allow iTunes to launch. Go ahead and try to resync the device normally and see if that fixes the problem of the missing files. If it doesn't, connect the iPhone again and then switch to the "Music" tab. Locate the grayed out files and drag-and-drop their matching counterparts from the iTunes playlist to your iPhone. This manually syncs the files individually, so be sure to allow iTunes time to finish syncing before disconnecting again.