SIP registrations rejected on startup, ok later

I have an asterisk box which has a sip.conf that calls for registration with 3 different sip providers. This box is restarted every day, after being switched off overnight. The startup instructions are in /etc/init.d/boot.local. When the box starts * appears to be ok, but ‘sip show registry’ shows that all 3 attempts to register with the sip providers were rejected. ‘sip reload’ at the * CLI (once the box has finished booting) results in successful registration with all 3 providers.

What would prevent the box from registering successfully on boot? Clearly it is not a question of an incorrect password or username.