ITunes will not sync music to my iPhone?

I've been trying all night to add Gavin DeGraw's music to my iPhone. The stupid iTunes I'm using (it's like 11.1.1 or something a bit lower, a rough guesstimate) won't let me add the music to my phone, no matter how many times I try. It's 54 songs, and I can't even put them in a playlist and sync a playlist.

what else can I do to put these songs on my iPhone? I've tried practically everything!

Added (1). I have an iPhone 4S, and I didn't buy the songs, I [downloaded] them. I did have a bit of success when I did something like unchecking the music and adding it, then rechecking it (big whoop) but that also caused the music files by the artist to be grayed out.

If you have an Iphone 5, the Itunes does not connect to your phone if you it is plugged into your computer. I believe that you can re-buy the songs you have bought before on the same account you had bought the songs on. I had the same problem with my phone as well. Also, if you have another apple product for example an iPod 4g or a lower generation, you can plug in your ipod first, then unplug it and then plug in your phone and that will most likely sync your phone to Itunes. Hope it works