Songs on my Iphone appear dark shaded grey and will not play?

I purchased some songs on Itunes, and when I try to put them on my Iphone, they appear a shaded grey and will not play, they'll just be sipped over, does anyone know the cause of this or even more importantly, the fix?

I'd erase the song listings from your iPod, exist and restart iTunes and try putting them on the iPod again. I'd use manual management of music and video, then drag and drop the files. That's what I dod when I have this problem. Hope it's your answer as well.