It is possible that it is a connectivity problem. Check that the server has connectivity with the device and that it can collect SNMP information through the terminal with snmpget, or using the SNMP browser of the Pandora FMS Console.
If the previous checks have given positive results, check the performance of the machine. If the Pandora FMS server has few resources or has a lot of load, it is possible that it is not able to perform the checks correctly.