If Network Discovery cannot get sysObjectID by SNMP v1, then this device will be ignored.
The issue is fixed in the IT Management Suite 7.6 HF7 release. Note that the fix works only when the registry keys that are attached to this article are applied to Notification Server. If corresponding registry key is set, then Network Discovery continues to send SNMP requests further even if sysObjectID is not received. SNMP version of requests depends on registry key. Set it to V2 to discover SNMP v2 only devices (use "force_SNMPv2.reg" file). This will not affect SNMP v1 only devices because it forces SNMP version only when sysObjectID is not received by SNMP v1.
Warning: Applying these registry keys slows down Network Discovery functions and communication with devices. Apply the keys only if you have in your network the devices with restricted SNMP v1, but with SNMP v2 support.