Usually the first place to look is the SNMP service. ITA does not directly communicate via 161/162, it just uses the service. Also, make sure you have open communication with 161/162 UDP. Setting the right community string and destination address in the service will usually get it done.
Here is an extract from the ITA UG for setting up SNMP on ESX servers and sending a trap :
Setting Up SNMP on VMware ESX Server 4.0 and Later For Discovery
1 Download the VMware remote command line interface tool (RCLI) from
the VMware website at www.vmware.com. You can download either a
Virtual Appliance, RCLI tools for Linux, or RCLI tools for Windows
2 Run the following command on the VMware ESX Server to configure
SNMP from RCLI:
vicfg-snmp --server <ESX_IP_addr> --username root --password <password> -c <community name> -p X -t <ITA_IP_Address>@162/<community name>
Where X represents an unused port.
NOTE: Multiple IT Assistant IP addresses can also be mentioned using a comma-separated list.
3 Stop the SNMP service if it is already running on your system:
Service snmpd stop
4 Add the following line at the end of the /etc/snmp/snmpd.conf:
proxy -v 1 -c public udp:127.0.0.1:X .126.96.36.199.4.1.6876
X represents the unused port specified above, while configuring SNMP.
trapsink <IT Assistant_IP_Address> <community_name>
trapsink is required to send traps defined in the proprietary MIBs.
5 Run the following command to enable SNMP for VMware ESX Server:
vicfg-snmp --server <ESX_IP_addr> --username root --password <password> -E
6 Run the following command to view the configuration:
vicfg-snmp --server <ESX_IP_addr> --username root --password <password> -s
7 Restart mgmt-vmware. This takes a few minutes before the service is started.
service mgmt-vmware restart
8 Restart the SNMP service.
Service snmpd start
9 To sustain a system reboot, start the snmpd service.
Chkconfig snmpd on
10 Run the following command to send a test trap to IT Assistant:
vicfg-snmp --server <ESX_IP_addr> --username root --password <password> -T
NOTE: Ensure that the SNMP ports are open before sending traps to the management station by typing
esxcfg-firewall -e snmpd