Fortinet not responding to MIB

1st is snmp enable on the interface and you have the correct  snmp-community or user/auth if v3?
 
Did you run "diag debug application snmpd -1 "
 
 
e.g 
 
  diag debug enable
  diag debug reset
  diag debug app snmpd -1
 
 
If your calling up mib buy name, I would stop and do a simple snmpget or snmpwalk against the device? Does it reply?
 
e.g 
 
  snmpwalk -c bahblah -v2c x.x.x.x
  or a simple sysname get
  snmpget -c blahbahb;ah -v2c x.x.x.x   .1.3.6.1.2.1.1.5.0
 
 
Ken Felix