Since I have had my S6 I have not been able to get the Android Device Manager to ring my phone. It locates it accurately on the map and will lock the screen ok, but it just wouldn’t ring. Not much use when you know the phone is *somewhere* in the house.
Today I did a bit of experimentation and discovered that my notifications was set to silent, changed this to a value in the middle by pressing the up volume key, selecting the settings gear and then changing notifications and now the phone rings even if the phone is set to silent.

Hope this helps someone else as it does seem to be a common issue with not many useful solutions.
Also, to turn on the android device manager capabilities on the phone, go to apps, settings, lock screen and security, other security settings, Device administrators, and ensure Android Device Manager is enabled (simple huh?)