Some songs won't transfer to iPhone 6

Hello, everyone.

I have an assortment of songs in my iTunes library. Some are songs I have purchased through iTunes, and some I have ripped from CDs, and some I have made myself in Logic. It seems that I am unable to drag songs from my iTunes library on my iMac to my iPhone when I have them connected. I don't know what the problem is, and I have searched the internets high and low for a solution, but no dice. Can anyone help?

Thanks!

Joe