I had the same issue with my iPhone 4s with the latest update of iOS 8. It started to happen when I bought songs from iTunes using the WiFi at work. The connection dropped unexpectedly and I was never able to download the song on my iPhone. It was in my iTunes library on my home Mac but, whichever method I tried, I still couldn't add it to my iPhone. After trying everything Apple support recommended to do, and most solutions posted here, I deleted my playlist, Restored the iPhone (after backing up) and it solved the problem. I recreated a new playlist and was able to add all the songs that appeared greyed out with the little dashed circle next to it, in the list.
Hope it works for you too, if everything else fails. Apple leaves me angry and frustrated, sometimes, but I'm still crazy after all these years.
Cheers.