SNMP to the VPN

i have 2 firewall one in HO and another in the branch . and a PRTG server in the HO . i allwo any to any for all network including vpn .

vpn is using site to site ipsec and every thing is working fine.

in enabled snmp agnet on the branch office vpn interface and it work fine i can get snmp traffic using a prtg in the branch site . but when i try to get the same snmp traffic using the prtg in the HO site i couldnot .

so how can i enable this ? as i said all service network every thing is allowed .