Verify the community string has been set properly, you can set this in server settings and probe setup screen.
Verify that the version of SNMP that you have specified matches the version your device is configured to use.
Check your devices to see if SNMP is enabled and what version of SNMP they support.
Some devices you have to specify SNMP version when you enable it. You can use an external tool called SNMPWALK. This will allow you to run SNMP Get or Get Next requests outside of Intermapper if you need to determine if IM is issue; however, if you are getting SNMP from your other devices, then probably not Intermapper, but a setup or device issue.