[Plugin: WP Super Cache] no cached pages and zero byte cache objects

Two (related) issues I need help with asap!

1. pages are getting run thru the super cache, but none get properly cached. I end up with zero byte objects in the cache

2. related to this, I see
<!– Dynamic Page Served (once) in 0.136 seconds –>
but never
<!– Cached page served by WP-Cache –>
or
<!– super_cache –>

The plugin manager tells me I have 5 supercached pages, so it seems the system is functioning but not actually caching the pages.

I have tried clearing the browser cache and cookies, and tried from multiple systems to ensure I’m not logged in.

I am assuming the issue has to do with something in my template or php header that is preventing the cache from storing the objects but have no way to troubleshoot it from here.