When i sync my ipod to my itunes the new music i put on my itunes wont show on my ipod

Recently i have been trying to sync my ipod 5th generation with my itunes and every time it syncs it doesnt add the new music i have put on my ipd that i have on my itunes, and also every since i have synced it some songs on my ipod do not play, they just skip over the song on my ipod, but all of my songs in my itunes on my computer play. Does any one have any ideas how i can fix this??