Ani to find your problem, pick on one song that wont play in your loaded list , locate it on your device copy and paste it to your ST3 folder refresh your database in song mode by pulling down you song list, and see if that song will play now.
It could be some songs are copy protected and wont allow you to transfer to ST3.
Peter will probably need more information from you to figure it out, like what is your device, software version you are running , the song audio type and where you are trying to load it from, itunes,another folder, icloud etc the more information you give the easier Peter will be able to help you.
Cheers Damir