iTunes library incomplete on iPhone/iPad I have about 16,000 songs in my iTunes library, but what shows on my iPhones and iPads in "My Music" is much less; not even all of my artists appear in the Library/My Music window.  I have the very latest updates on all devices, including iTunes.   Truly vexing....
You will need to go to Music, then select the drop down that says, "Songs" or "Albums" or whatever your current library view is. In the bottom of the drop down menu, switch off the option that says "Only downloaded music", then you can download the music that you're missing from there. If that fix doesn't work then you might have an iCloud syncing issue.