[exim] randomize not working as expected

Hey guys,

I am currently responsible for the administration of four exim servers
(running 4.66) which we utilize as our inbound MX servers. Mail for our
domain is then manually routed via the exim machines back to our main
offices using the manualroute option over a compressed ssh tunnel. I'm
using the randomize option to give me crude load balancing, but it
doesn't appear to be working very well. What I'm seeing is that one or
two of the servers at our main office seem to be getting much more load
than the other two.

I am using the following router:

  forcepath:
    debug_print = "R: forcepath for $local_part@$domain"
    driver = manualroute
    domains = !+local_domains:partial0-lsearch;CONFDIR/forcepath.list
    route_data = ${lookup{$domain}partial0-lsearch{CONFDIR/forcepath.list}}
    transport = remote_smtp
    host_find_failed = defer
    same_domain_copy_routing = yes
    self = send
    no_more

And the following transport:

  remote_smtp:
    driver = smtp

Here are the contents of forcepath.list:
domain.co.za: 127.0.0.1::10025:127.0.0.1::10026:127.0.0.1::10027:127.0.0.1::10028 randomize byname

Can anyone help me to get these machines to balance more equally? What
am I missing?

Thanks in advance,
Marc