I've scanned the PCs of my network using first the Domain Scan. After fixing some problems now it scans the PCs correctly and shows them as part of the domain.
Some computers aren't scanned because they don't appear in the Computers list of the Active Directory, so I decided to scan them (Windows mainly) using the Network Scan, giving the domain credentials of the administrator.
It has detected already (detected previously with the Domain Scan) and new computers. I don't know why, but some of the computers which where detected using the Domain Scan and where identified as belonging to the domain now they appear outside the domain.
I can't understand why the computers that were correctly scanned using the Domain Scan and belong to the domain, after the Network Scan they are outside the domain.