I see several posts on this, so I'll add mine..
We're running a Standalone Evaluation of Asset Management, and
we've noticed some of our machines aren't showing up as scanned
in the Web Console after installing the ZAM client, others don't
show up as having been scanned on the correct date even though
they have been rebooted, had the service restarted, or had a scan
forced, etc.

Neither server or client have firewalls running at the moment.

I put a client into diagnostic mode and also searched the client
and server logs, but I can't find the problem. I'll gladly post any
of those, and would like to determine a solution fairly quickly
as we only have a few weeks remaining on our evaluation.