My best guess is you are triggering digests on size (Digest options -> digest_size_threshold) and either not periodically (digest_send_periodic = No) or cron/senddigests runs only weekly.
To clarify, he has set digest_size_threshold to 10000 and digest_send_periodic =Yes. /usr/lib/mailman/cron/senddigests is run by cron every day at noon.
Other digest settings is
nondigestable = True scrub_nondigest = False digestable = True digest_is_default = 1 mime_is_default_digest = 0 digest_size_threshhold = 10000 digest_send_periodic = 1 digest_volume_frequency = 3 discard_these_nonmembers = (241 members) ban_list = (188 members)
The digest_size_threshhold = 10000 may likely course the problem, but is necessary as a digest containing a single mail is sent each time a mail is sent to the list-address as suggested by https://mail.python.org/pipermail/mailman-users/2014-August/077622.html.
Regards Henrik