Why can't I copy songs? ...

We have OSX Yosemite, and we have 5 iPhones in the house (4 5s models, and a 6).

We have a combo of songs purchased via iTunes and music we imported from CDs we own.

Frequently I simply can't get iTunes to copy songs to our phones. Many times I will attempt to drag music from the library to a phone connected via Lightning cable. Either nothing happens, or the iTunes status flashes quickly and the song never gets copied. The only way I've been able to get around this is to delete every single song on the phone and start over. This usually works, until a few weeks later when I connect a phone again and have the same problem.

Any idea what the problem could be? I've been using iTunes on Windows then Mac for many years and this type of problem seems to be getting worse.

Also, often when I plug in my phone and then look at the music on the phone via iTunes, about half of the songs are grayed out and half a dashed circle in front of them. What does that mean, and why does it happen?

Thanks very much in advance

p.s. I never want to automatically sync anything, all I want to do is drag songs manually from iTunes to my iPhone.