Music syncing problem

I recently bought a laptop, I've been syncing my music, videos, etc. tob my iPhone 4 manually because I don't have enough free space on my phone. It synced fine on my home desktop, When I tried to add videos or music to my phone manually on my laptop, it won't allow me to. Whereas I would usually get a blue + when I drag the content to the phone, now I get a red circle witth a line through it. How do I fix this?