All of my songs are not synced in I-Tunes with my I-Phone 4S - why??

De-sync all music to remove it from your phone. Then reboot the phone (hold down the home button along with the power button until you see the Apple, then let go), then re-sync with iTunes.

Also check if you are using the same account info on your computer and on the phone in Settings/iTunes and AppStore/Apple ID.

More info here:

iTunes Store: Associating a device or computer to your Apple ID

About iTunes authorization and deauthorization

Go to iTunes, Click on your phone, click on music, click sync music, click selected artists playlist etc., now do not check any of the boxes. Then sync, and all the music on your phone will be removed.

Go to Settings/iTunes and AppStore/Apple ID, sign out, restart the phone and sign in again. You can only sync music bought with your Apple ID, or music you copied from your CDs.

Thanks Amishcake, it worked for me! I had two songs that for no apparent reason stopped syncing to one of my playlists on my iPhone 5 though iTunes was telling me they were there. So glad I didn't have to restore the iPhone and lose various app data.