There can be an ASNMP request failure during discovery.

You can confirm that by looking into the "Description" field of the pending list (AD_PendingData):

If it shows any one of the following or both the error messages in the "Description" field of the pending list (AD_PendingData) , then this is due to any ASNMP error occurs during discovery:

"ASNM-E-ASNMP_SEND_RECEIVE_ERROR-ASNMP request failure: Error when sending or receiving SNMP data for Host '<IP address of the device>' (Maybe bad credentials)."
"ASNM-E-ASNMP_SNMP_ERROR-ASNMP request failure: Error <Error Code>: <Error Message> when receiving SNMP Packet from '<IP address of the device, port<port number>'"