hey siri does not work on iPhone 6+ or 6 ?

Found out that hey siri must have been activated in 9.3.2

My wife old 5 had it flash up on screen.

Activated it on hers and it works fine

It also works on my iPad mini 4

It does not work on my iPhone 6+

or my sons iPhone 6

or friends iPhone 6

Siri itself works fine just not hey siri.

Any ideas please

thanks