We might need more of your configuration output to try to help you.
If you've got the lo0 interface configured and have some deny terms in firewall section, you should allow there your monitoring host for traffic to snmp port.
And you might need the security policy for the trust zone allowing snmp traffic to it. AFAIK the default policy is deny.
Sou you might need something like
set security policies from-zone trust to-zone trust policy permit-adm-and-routing match source-address mon-host set security policies from-zone trust to-zone trust policy permit-adm-and-routing match destination-address any set security policies from-zone trust to-zone trust policy permit-adm-and-routing match application any set security policies from-zone trust to-zone trust policy permit-mon-host then permit
and add the address in address book for mon-host.
Also you might have the junos-host zone (or smth similar), so the policy could look the seam but to-zone should be junos-host