Reduce the valve spring length or stiffness, or increase the tank output pressure.
I would look over the cupseal and confirm it is screwed on completely and not loose. I used a small piece of electrical tape on the threads to jam it in place. It is removable, and will not chemically attack the cupseal. (I don't know if Loctite or similar chemicals will interact badly with the cupseal plastic, so it was safer to do it that way.)