I have set up a Google play music sub and I would like to listen to my music in offline mode so can listen without any internet.

But I must be doing something wrong as when I turn my internet off and go into my playlist not all my songs let listen offline but only about half of them.

So what am I doing wrong? or is it only some songs that can be listened to in offline mode?

They haven't all synched with your phone. It transfers to your phone when you listen to it, unless you do it manually.

That's my experience of it anyway, you could try it with one song.
I wanted to be able to listen to my songs over data without them loading onto the phone as i only have 8GB storage.