Try washing it with soap that cuts grease. The fat residue from breastmilk may be causing the valve to fall off. If the problem persist after washing, it's probably time to replace it.
Try buying the Maymom valves. Much better suction power too. Also avoid drying the valves in a uv sterilizer as the heat from uv spoils the valve
Valves shld be changed once a month. You can simply purchase it online and replace the valve.
Change to the medela valves - works much better.