Why doesn't my iphone sync new songs?

I have songs in the library waiting to be synced, but whenever i click file on top of itunes, the sync option is gray? Why can't i sync new songs even if they are in my library?

You may try iPod/iPad/iPhone transfer software, it's a nice iTunes alternative that can help you upload music and video to iPod/iPad/iPhone, I always use it to sync my iPod when my iTunes can't recognize my iPod, it's easy to use and works pretty well