DG 940 will not replenish tank after steaming?

Can anyone tell me where I can geta sensor or part to fix my Rowenta DG940 steam generator. When first switched on it fills the pressure tank and after reaching correct pressure goes into standby for use. On pressing the trigger button on the Iron, the steam releases but it is not rplenished into the holding tank. It should automatically refill the pressure tank, but this does not happen. Only by switching the machine on and off, will this start the process again. Can you help?