Clean it well and ensure you are blasting air pressure into the tank quickly so that it closes effectively. If this does not work I would suggest using a magnet, suction cup, or even a screw to manually lift it before you prime the abrasive tank with air.

Make sure the plunger is aligned with the opening. Have a ball valve in line as close to the tank as possible. this will allow a large volume of air to force the plunger into position.