Multiple routers behind the same Firewall and IP address

We are trying to capture netflow from 4 different routers (with multiple interfaces) behind the same firewall. We have enabled netflow on them and some of the interfaces appear in the analyzer under the 1 public IP address of the firewall.

It appears though if the ifindex number is the same or duplicated on multiple routers we only get 1 of each name into the analyzer. For example if ifindex1 is on 2 routers we only see 1 interface not both of them and its also hard to determine which of the routers the interface represents.

How can we get all of the interfaces on each router to show up in the analyzer?

thanks,

Dimitri