If SNMP hasn’t yet been enabled on all or some of your network devices, then your network diagram will not display the proper hierarchy. Further, enabling SNMP can be a helpful troubleshooting tool. In the device matching page, you can see if SNMP has worked for a particular device and toggle the settings to work towards a solution. Please refer to our Guide to successful Network Glue deployment KB article for details on SNMP.
Also, check if you have Group Policy (GPO) configured in your Active Directory (AD) on your network. If so, note that you will be able to enable SNMP only ones for all devices throughout the network GPO. If you do not have GPO enabled, you will need to enable SNMP for every device separately.