There was a thread on similar topic about a week ago that was recently closed.
ZA was having trouble with their servers, but claimed it was resolved .... apparently not...
I have been having the same problem since upgrading to V9 as well.
Usually I can force the update to work manually if I retry several times.
I like to make sure I have the latest signature files before pulling in the morning's mail but this monrning I can't get an update at all --