First, thanks so much for all your work on this plugin -- it's fantastic, and a pleasure to use.
I have two instances of the same code base on two different servers (dev and QA instances). The same file permissions are in place for both. Both have the same plugins installed and enabled, and as far as I can tell, the W3TC config is the same for each. I have debugging turned on for both.
The issue: Although I have page caching, DB caching, and minify turned on for both, only one is doing the page caching.
The debug info for one instance includes the three sets of comments (minify, DB cache, page cache), and populates the pgcache directory with what you'd expect. On the other instance, the comments only include minify and DB cache. No page cache comments, and no files in the pgcache directory.
Both instances serve up identical content, and neither is registering anything in the error logs. Neither WP admin settings page shows any errors or warnings. Each site is running PHP 5.2.4.
Can you think of anywhere else I would look to debug this problem? Anything that might be different between the two servers? Would the presence or absence of any PECL libraries, etc., silently defeat page caching?
Thanks again for all the work and community support.