Backup GC only working on some sites

https://gist.github.com/anonymous/5938329

Since the beginning of April (2.0.6?) I've had a bunch of my sites not having their backups garbage collected. There are still a few sites that the GC works though. I've made sure the site settings are the same (below), and a couple are even clones on the same platform and one doesn't work.

client: admin
Backup schedule: default
Scheduler status: backups enabled
interval: one day
cron interval: disabled

Is there a log I can check for errors, or any settings to check?

thanks!