By default, an asset is only generated for an IP address if data can be pulled from one or more of the following protocols on the device: FTP, HTTP, HTTPS, JetDirect, MOB (Managed Object Browser), SIP, SMTP, SNMP (SNMPv1, SNMPv2 or SNMPv3), SSH, Telnet, WMI. If none of the aforementioned protocols are enabled, no asset will be generated, even if the IP address can successfully be pinged.
You can tick Save Pinged IP for your ranges under Configuration\Scanning Methods to force assets to be generated for all pingable IPs, but no device data will be available unless a protocol is enabled on the device. More info on network device scanning requirements can be found in
this knowledge base article.