MIB file is used to interpret the OID names to their corresponding OID values. MIB files varies with each versions of the hardware.
Both OIDs and the names will works if the MIB file used is the correct one. When the node name is not available in the MIB, or when it cannot read the MIB file, it will throw the exception 'Unknown Object Identifier'.
Sometimes, new managed objects will be defined for a recent version and it will not be available for the old versions. But the OIDs will not change. Each versions will have the unique OIDs if its a new object and the existing OIDs will not change.
That's the reason, you are getting the response while using the OIDs and not when using the names.
To resolve this, make sure you use MIB you got bundled with the version of your product.
Hope that helps!