Hansgrohe shower

Question

Hi, just fitted a new hansgrohe thermostat concealed shower unit to a gravity system using existing pipework thoughly flushed both hot and cold sides through the Ibox then fitted the thermostat unit but when i try to turn the the temperature to the cold it just shuts the cold water supply. So no blockage getting to the Ibox do you think the non return vale within the thermostat unit could be faulty bearing in mind this is a brand new shower unit