I cant drag and drop any music from my itunes to my iphone 4s.

Every time I try to drag a song to my iphone from my itunes library it comes up with a red circle with a line through it and wont sync. I have my settings set to manually manage so I dont know what is wrong. I dont want to create a play list as then I will lose all my music and have to load it all up again on a playlist... I have tried putting one song into a play list and dragging that over to my iphone but that doesnt work. help!