When the pressure shows zero it means you are not getting water into the pump.
Another possiblity (from not filling hose first) is that your vacuum hose is collapsing and restricting the flow to the pump. In my pool, I have to adjust the 3-way valve so I am drawing half the water through the main drain and half through the skimmer (where vacuum hose attaches) when cleaning the pool. If I let all the water (suction) go through the vaccum cleaner the hose will collapse.