[exim] Router not called - mystery

Hi,

This router consistently does not get called for some reason for
particular types of email:

large_media:
   driver = accept
   condition = ${if and { \
        { ! def:acl_m6} \
        { def:acl_m7} \
        {! def:h_X-Cheetah-MedQuar:} \
        }{1}{0}}
   headers_add = X-Cheetah-MedQuar: True
   transport = large_media_mail

I run the mail through this test below:
# exim -d+expand+acl+uid -bh $IP_ADDRESS < email.txt >out.txt 2>&1

And the result is:
check set acl_m7 = 1
While acl_m6 is not set.
The 3rd condition also is true. The X-Cheetah-MedQuar: header is not
defined.

The router that finally handles this mail is right after this router. So
there is no problem with a prior router handling the email.

Any idea why this router would work 95% of the time. How might I better
investigate this?

Thanks very much.
Craig Jackson