I think I found the reason. And a possible fix...
For some reason in the plugin... the default value for the miblist is commented out. Effectively the -m parameter ends up with a '' instead of 'ALL'.
the quick fix is to hand-edit the check_snmp.c file in the nagios-plugins-1.4.11/plugins folder and change this line:
/* miblist = strdup (DEFAULT_MIBLIST); */
miblist = strdup (DEFAULT_MIBLIST);
& remake/reinstall it.
That should be on line 178.
Does anyone know why it was left commented out?
Alternatively... you can specify the "-m" parameter & put "ALL" in there if required.