When actively scanning a domain, why would some machines not be discovered?

[Prodigus]

Why would some machines be discovered when pointing to a domain (specifically one of the domain controllers)? When a domain controller isn't discovered it makes it very obvious that you missed it but now I'm concerned that if that one is missing that I might be missing others as well?

Is this a common problem and something I can tweak on my eand to help with this? Do I have to manually add it?

[Hemoco]

Active scanning scans for the last logged on date for each computers. Some workstation report in every 15 minutes, member servers once a day and Domain controllers sometimes longer.

Do you see any errors in the errorlog.txt file?

[Prodigus]

Yes.. I am referring to active scanning. I'd say it's been running 4-5 days...

I notice if I just go in and add it to the scheduled scanning under the Scan Options then it will actually scan it but then that still leaves me with the question of if anything else is getting missed with Active Scanning.

Great tool so far.. I'm just trying to work out some wrinkles.

[Hemoco]

Are you referring to Active scanning?
How long is active scanning running?