I have a great proble in one machine (in the others all works fine) when I connect with the machine remotely by vnc or putty and i start the asterisk, when i close the putty sesion or the vnc sesion, the asterisk service closes.
Maybe there is not a startup script for asterisk in that machine so when you close the session the asterisk crashes. in the asterisk source directory there is one called contrib in wich you can find a script to safely and backgronu run asterisk for different platform.