Only some songs are added to iphone. enough space on phone

When adding song from itunes to iphone, some songs are successfully imported, whiles others are shown (both in itunes and on my phone) but they aren't completed. In itunes they are grey and have a weird circle next to them and on the phone they are also grey but have a red circle with a "stop" sign in the circle. I can't access them. Also I have checked and there is enough space on my phone for all songs. Can someone help me?