Some songs won't load onto my iPhone 4 D:

Please, I put about 300 songs in my iphone

they all got sync., but when I want to add some more, they won't ''load'' in it; the little circle isnt fulled like the other songs did. I have enough space in my phone, and I already tried to put away all the songs and put them back. I closed my phone many times, and my computer too. I'm almost raging. help me.