Not all subnets are balancing

[Norman Elton]

I've got two DHCP servers configured for failover:

failover peer "wm-dhcp-01-02" {
  primary;
  address 1.2.3.4;
  port 647;
  peer address 1.2.3.5;
  peer port 647;
  max-response-delay 30;
  max-unacked-updates 10;
  load balance max seconds 3;
  mclt 1800;
  split 128;
  min-balance 60;
  max-balance 300;
}

I've noticed that our DHCP log file shows helpful information when
subnets get balanced:

balancing pool 7f9da82e7960 INIT-WM-HALL  total 240  free 88  backup
96  lts -4  max-own (+/-)18

But this doesn't seem consistent. That is, there are some subnets that
balance, others never show up in the log. Presumably there's some
small difference between the two, but I can't figure it out. Here's a
subnet that is getting balanced:

group {
    option subnet-mask              255.255.255.0;
    option domain-name-servers      128.239.29.9, 128.239.20.9;
    option domain-name              "wm.edu";
    default-lease-time              3600; # 1 hour
    max-lease-time                  3600; # 1 hour

    shared-network INIT-WM-HALL {
        subnet 10.45.170.0 netmask 255.255.255.0 {
            option routers                  10.45.170.1;

            pool {
                failover peer   "wm-dhcp-01-02";
                range           10.45.170.11        10.45.170.250;
            }
        }
    }
}

And here's a seemingly similar subnet that is never showing up in the log file:

shared-network ADMIN-NET {
    subnet 128.239.96.0 netmask 255.255.224.0 {
        option subnet-mask              255.255.224.0;
        option routers                  128.239.102.1;
        option domain-name-servers      128.239.29.9, 128.239.20.9;
        option domain-name              "wm.edu";
        deny                            bootp;
        default-lease-time              432000; # 5 days
        max-lease-time                  432000; # 5 days

        pool {
            failover peer   "wm-dhcp-01-02";
            range           128.239.102.10  128.239.102.254;
        }
    }
}

Am I missing something? Is there some reason why the first would
rebalance and the second not?

Thanks,

Norman Elton