Try a tiny drop of gun oil like bisley in the cylinder where the pump rides up and down for the pump seal to prime itself, it may be bone dry. It has happened to me before bit I have taken the gun apart so many times that everytime I do it gets a service. Alternatively your pump head may need adjusting slightly longer as it may be too far apart from the valve as the arm is fully shut? If the drop of oil doesn't help, let me know and I will try and rally you through it.
Daniel