My hp470 will not run?

my hp470 lawnmower will not run, it has fresh petrol, a new plug and a clean air filter, i have checked the carb and all appears ok unless i am missing something, when you spray easy start into top of carb it will start but dies straight away as i suspect not getting petrol through to engine, on my user manuel it states when pushing the red prime button you should see a spray of petrol going into the engine but i do not, am i looking at a problem with the diaphram or is it something else, where do i start ??