Alexa will not link fully to Hue bridge

[Goldian]

HI - I am hoping you may be able to help with this very simple problem with Alexa and Hue linking

I have successfully had Alexa discover my 4 hue lights and bride (the latest square one) and alexa can switch on and off and bright and dim, but changing colour or scene doesn't work

It seems that the Hue skill is not enabled, but when I go through that process on my iphone, press enable skill, log in to Hue on the phone (and the phone alexa and bridge are all on the same wifi), when it asks to press the central button on the bridge - there is no response on the Alexa-Hue app ?

Anyone had this problem - and if so could you let me know how to fix it ?

many thanks indeed
Ian