Lansweeper 5.1 will only add a device to the database if it can pull data from one or more of the following protocols: WMI, FTP, HTTP, HTTPS, JetDirect, SIP, SMTP, SNMP, SSH or Telnet. You can run Program Files (x86)\Lansweeper\Actions\DeviceTester.exe on your Lansweeper server and connect to some of your devices to see which protocols are enabled.
In Lansweeper 5.2, you can tick "save ping IP" for your IP ranges to have Lansweeper create assets for all pingable IPs, though no scanned data will be available unless you enable one of the aforementioned protocols. "Save ping IP" can be enabled in the IP Address Range Scanning section of Configuration/Scanning Setup/Scanning Methods.