Reset pulse ISP mode for LPC932 only once !

[Sly]

Hello,
I'm using the "Reset pulse ISP mode" for programming an LPC932 soldered on a PCB. I've made a hardware dongle as described in the AN10221.
It works the first time (new chip) : I press the button, the led turns to green, Flash Magic starts. My chip is programmed.

But it fails the next times. After the led turns to green, the previously loaded firmware runs instead of waiting for FM to program the flash.

The delay between power-up and the rising of the reset pin is 12.4 ms
the high state duration of the 3 pulses is 20 us and the low state duration is 10 us. It seems to match with the specs.
What could be wrong ? Thanks a lot.