I don't have an answer other than to say that the AVG client sometimes fails to write scan results to the central database. For example I have a network where the PCs do update the scan results but the server doesn't.
If you look in "C:\Documents and Settings\All Users\Application Data\avg8\scanlogs" on the PC you should see the scan logs, though they're binary files so not much use. However the last modified time at least tells you that the scan ran and when it finished.
On the PC running the Admin console you'll find a SQL database called avg8, and if you run the query:
select * from avg_scanlogs
where avg_scanstarted > '2009-07-01'
order by avg_scanstarted desc
it will show you what data is in the database. In the case of the network mentioned above the server scanlogs aren't in the database even though the log files are present and the scan takes a couple of hours so it's obviously not bombing out.
Actually, I've just had a thought. I'll have to look into this, but it may be that the scan results are only getting written to the database if someone is logged in. I'll the aforementioned server logged in tonight to see if the scan results get recorded.
Edit: leaving the server logged in made no difference. It still did not log the scan results to the database. As Cameron says in his comment, requesting the scan results apparently works but fails to send any data. It's down to a bug in AVG8.5 I guess.