iPhone 4s Music Syncing problems.

iPhone 4s connected to iTunes by usb, and when i synced it about a week ago, it removed all my music content. I checked that it still had the box checked to sync all my music, and it was. After restarting my phone and the computer, and tried to sync it, the music was transferred to my iPhone. The next day when it connected over wi-fi to itunes, it removed all my music once again. Now it wont transfer even though the correct box is checked under the music drop down of the iPhone section.

When i uncheck and recheck the 'Entire Music Library' it does show how much storage the iphone will have syncing the music across (and there is still available space after it would be transferred so it can't be that there is not enough storage), and after hitting apply and sync, the music does not transfer across, and the bar showing storage at the bottom goes back to showing no music in it.

How do i get it to sync properly?