“Blog pages show at most” in The Loop

The only reasonable guesses that come to my mind are that the theme (or a plugin):

1. Modifies the main Query with a filter on pre_get_posts, so look
   for that filter.
2. Just creates a new Loop with new WP_Query($args). You will have to dig through the template file to find out.
3. Interrupts the post_per_page save process or alter the
   posts_per_page option in the database, so check the *_options
   table for the posts_per_page key (option_name) to rule that out.
4. The Loop is not calling the_post. Without that, the Loop will
   never increment to the next post and the loop will run until the
   browser gives up.

I’d bet on #1, then #2, with #3 being a distant third. Without more information I can’t give a solid answer.

Item #4 is a very viable option but the problem is not the query itself but the Loop.