Thanks for providing the information requested. There are a number of things which could be causing your problem.
There are two basic reasons why a request may go unanswered. Either the management application does not like the request so never sends it, or the agent does not like the request so never responds. The simplest way to distinguish between the two is to run snmpwalk with debugging enabled and look at the output. Hopefully your version of snmpwalk allows you to enable debugging output.
Other possibilities to consider:
The agent may return a response to the original query but the management application may not like this response, and refuse to display it. The typical symptoms of this would be that the debugging option would display a sequence of sending and received packet dumps, with the same contents each time.
The agent may simply not support the MIB objects being requested. This is most commonly seen when using snmpwalk. Debugging output would show two pairs of raw packet dumps - one a GETNEXT request, followed by a GET request.
The agent is taking too long to respond and timing out (unusual)
Poorly implemented enterprise MIB
I believe that RHEL, on which CentOS is based, uses the NET-SNMP toolset (
Net-SNMP). There is an excellent FAQ there which will assist you in further debugging your problem.