Itunes doesn't copy all my songs to my Iphone?

I previously removed some songs from my phone, but I wanted them back on my now. So I clicked autofill and strangely not only did it not put all the songs back, it even removed some others. I tried selecting all my songs and dragging them to my phone, most of my songs are there now but still not all of them. I'm not sure what went wrong here.

Are you out of space? Or did you select sync entire library?