iPhone won't sync most music with Macbook Pro

I have had a Windows Vista PC for some time and being finally fed up with Windows I decided to switch and got a Macbook Pro.

With the new Macbook, I connected to my Windows PC and copied over all my media. I can now access all of my music, videos, movies and TV shows on iTunes on the Macbook. However, when I synch my iPhone with the Macbook's iTunes, almost all the music disappeared from the iPhone. I'm not sure why certain songs stayed, though I think they may be tracks I bought on my iPhone.

After trying to sync with select playlists, artists, albums, etc and no success, I tried to tell it to autofill from a playlist (called iPhone, this is what I've always synced to my iPhone), I got a message that I couldn't autofill because none of the songs in there (about 4,700) can be copied to the phone.

Does anyone have ideas why this may be the case? Both the iPhone and the Macbook are signed into the same iTunes account. Most of the songs I've bought via iTunes will not sync despite this. Additionally, songs I burned from CDs over the last decade or so (so no DRM involved) will not copy either.

Any help or advice is greatly appreciated.