Depending on the type of device (Windows, Printer, NAS, etc.) LanSweeper has different ways to access the serial number.
If it's a Windows Device, make sure that the LanSweeper server can access the ADMIN$ share from the scanning server using the credentials for that IP range. You could log on to your LanSweeper server as the Windows scanner user and access the Windows computers UNC for the computer (i.e. \\COMPUTER1_NO_SERIAL\ADMIN$) and get a connection, it should be able to access the information. You can also use the Windows Connection tool on your scanning server at C:\Program Files (x86)\Lansweeper\Actions\testcomputer.exe and enter the computer name, username and password to see what error, if any, occur
If it's a Linux device, make sure the SSH credentials and/or SNMP are set up correctly and that the device firewalls allow that access from the LanSweeper server. To test SSH, use C:\Program Files (x86)\Lansweeper\Actions\putty.exe and connect to the linux device with the credentials you set up in the Scanning Credentials.
If it's a device that LanSweeper scans via SNMP, make sure the SNMP community string (for V1/V2) in scanning credentials, or authentication (For V3) match and you can access in LanSweeper. You can test this by using C:\Program Files (x86)\Lansweeper\Actions\devicetester.exe
