Auto Scan when mounting a volume won't start

I've been using NAV Mac for about 2 years and it is configured to automatically scan newly mounted volumes which it has done properly since installation. For some reason this no longer occurs. I checked the Auto Scan preferences and they are still good. 

I'm on a recent Intel based Mac running OSX 10.5 and have all NAV updates installed.

Any ideas how to get this working again?

Thanks

Jerry