Hello,
the blank chip is using the forced monitor mode, there is no high voltage needed on IRQ.
The programmed chips need Vtst apllied on the IRQ Pin. Sometimes the problem is that the voltage of the MAX232 charge pump is rising too slow to get Vtst applied early enough in the reset sequence. A workaround is to put a delay into Power on Reset. You can do that by keeping a reset button pushed or by adding a RC combination to the reset pin. This works fine for me.
Eckhard