when i connect my ipod to my car stereo it wont play all of my songs like most of them arent even on there. whats up with that?

when i connect my ipod to my car stereo it will play the music from my ipod just not all of it, i have 288 songs on it but only about 50 of them will play, i dont know if its how i add them to my ipod or if its the format of the songs or what. i thought because it came from my itunes that all of my music would be in the correct format for my ipod. i can listen to all of my music with head phones and they are all there untill i plug it into my car stereo. please some one let me know some thing