"Not scanned" items does not stay in Assets (when missing 1st scan)

[Kennedy]

Hi,

I have around 450 machines and not all of them are being scanned. Around 60 of those are outside out network and they can't be scanned.
( First time scanning is missing. I reinstalled scanning server and started new DB. Old one had problems with db search. No problems with atlease once scanned.)
I have notice that once a day "Not scanned" computers are gone from Assets and they re-appear at noon when day scanning starts.
All machines are active in AD.
I don't have cleanup settings other than 5year+ computers are gone.

How do i keep those machine visible and "Not scanned" state all the time? They will be scanned someday.