Songs are dissappearing from my iphone 6

I've noticed that sometimes when I download new versions of software and my phone syncs, Music that I've previously bought from Itunes disappears from my phone. It's still on my computer, but I can't get it back on my phone, because when I try to drag it back over, nothing happens.

I don't know what my settings were before hand, but I've checked "Manually manage music and videos" and I still can't drag it over.

This is a historical problem that has happened for me on my old phone (4) as well. I'm not sure if it's a bug or incorrect settings, but here's all I want to have happen:

1) Music that's already on my phone stays on my phone no matter whether or not I do an upgrade to a new software version.

2) The only way music gets removed from my phone is if I delete it.

3) If I want music on my phone, I drag it from my itunes library to my phone and it puts the music there.

Anyone have any ideas why I can't just have the above 3 things work for me?

OSX 10.9.5

Itumes 12.2.1.16

Iphone 6 8.4.1