How to debug an empty cache_page table
Whether or not a page is cacheable is determined by drupal_page_is_cacheable().

Set a breakpoint there and dig into call stack / step through the conditions to determine the reason why nothing is saved to the cache_page bin.

Also take a look at drupal_page_footer() which calls into drupal_page_set_cache() in order to store the built page in the page cache.