I've pressed 'sync' in file and it says it's doing it, then sync completed, but the songs aren't showing up on my iPhone playlist? I've never had a problem with just pressing sync before

Phone and iTunes are both the latest updates.

iTunes > Your iPhone > Songs (or whatever it's called)

Make sure it is ticked to sync new songs...

All that is ticked, nothing new is showing on my iPod it's all my old music.

Edit: I just pressed something called 'Autofill' and that seemed to work. I've never had to do it that way before, just pressing sync used to work fine