Maxxforce 7 DPF problem

hi, my customer have an international with a maxxforce 7 and the problem is that when we want to clean the particul filter with the parked regeneration, it didn't work.
the procedure to do this task is to star the engine at idle, no engine temperature needed, parking brake apply, transmition at neutral and finally when all this is OK, just press the parkd regen switch.
Normally, the light on the switch goes ON, but mine is flashing like 4 or 5 times and turn off, no regen have started.
I dont know where to star to find this problem. I've got the international servicemaxx software and i can do a forced regeneration with the computer, but my customer said that by the time he bought his truck, he had never did the regen cause the switch didint work. his truck is a 2007.

i'm sorry for my perfect english, i'm originally french, thank you.

Best regards, Tommy