why I can't break

Hi all,
I've a target packet with arma.
I can't understand the following behavior:
if i set a breakpoint to writeprocessmemory from the start
and i hit f9 it doesn't stop and the program starts as
without the debugger.
if i set the breakpoint after some code stepping
(for example after stopping on a CreateProcess)
i can break in writeprocessmemory.
why does it happen?

thanks,
and cheers.