Array not working for archive page

Hello, I’m very new to this forum, so I hope this is the right place to post my question.

It seems like array command in my theme is not working, so I cannot list all of posts for monthly archive.

For April 2015, I had 23 posts, but it only shows whatever number of posts I selected in “Settings > Reading Setting > Blog pages show at most #”. I have selected 10, so it only shows 10 posts, and not even wp-pagenavi doesn’t work to show older posts.
Page: https://innadeshikoway.com/?m=201504

I’m not very good with array, and I cannot even spot the line to look into this. I hope somebody can help me on this, and greatly appreciate it.
Thank you in advance!