LPC84x Watchdog does not always work

We are having problems with the watchdog on a LPC845.

We are trying to force a watchdog reset on purpose because there is no other way to reset the LPC845. 

The problem:

On some hardware it works, on other equal hardware it simply does not trigger. Out of 50 PCB's we have about 10 failing on watchdog reset.

our code:

__disable_irq(); //Disable Interrupts

SYS_PeripheralEnableClock(SYS_PERIPHERAL_WWDT);
SYS_HardwareBlockPowerUp(SYS_POWER_HARDWARE_BLOCK_WDTOSC);

// The LPC_WWDT_T->TC value after reset (0xFF) is fine:
// that is the minimum value
LPC_WWDT->TC = 0xff; // not really needed

LPC_WWDT->MOD = 0x03; // Enable the watchdog and make it generate a
// chip reset at timeout

// Start the watchdog
LPC_WWDT->FEED = 0xAA;
LPC_WWDT->FEED = 0x55;

while (1); // on some hardware this triggers watchdog reset, on other hardware it simply stays here forever.

Hope someone knows a line of code that is required for getting the watchdog to work all the time...