Why do songs disappear from my playlists, and some never sync from my iMac, but others do?

I had to try multiple times to get 3 new songs to sync to my new (came out mid July 2015) iPod Touch. Finally, they did. Then a day later I noticed that one of them was gone! Where did it go? Why did it leave? This time instead of hooking up to my iMac again, I checked the Recently Added playlist, and there it was, so I just moved it to the playlist I needed it to be in.

Why does this happen? Are there settings somewhere that I have set wrong or mixed up, or something else?