Siri not working on iMac

I have recently installed Sierra on my iMac and are having issues with Siri. The main issue is that whilst Siri is enabled I can't get her to hear me. I have double checked the microphone (internal) which is working and enabled, I have double checked Siri is enabled, I have turned Dictation on and off and I have done the same with the computer without joy.

I know the microphone works as it works fine with Skype etc, I simply can't get Siri to hear me. Help please.