Following the move, whatever service is writing to the file will need to be restarted as well, for example:
system("/etc/init.d/syslog restart") == 0 or ...
I've seen situations where the application would continue writing to the old file until the service was restarted. Odd but true, and it might explain some of what you're seeing.
If you can, check /etc/logrotate.conf or any of the files in /etc/logrotate.d, and you'll probably see where certain services are being restarted following the rotation.
If things get any worse, I'll have to ask you to stop helping me.