I can get a few objects (OIDs) without problems but when I want to get more at once the API returns "null", why?

When sending a small GET or SET request with only a few variable bindings, everything works as expected. But when sending more than some probably not fix number of variable bindings at once (e.g., 50) a null response is returned by the SNMP4J API.

Answer

The agent (i.e., command responder) is not most likely not capable to process the PDU size that SNMP4J sent to it because you specified so much variable bindings.

However, the agent should have returned a PDU with a tooBig error. Not doing so, violates the SNMP standard and should be fixed on the agent side.

To workaround the issue, simply send less variable bindings at once.