Category page show wrong posts when load more

I have create a category page for posts under category “iOS”.
The page template is “Blog-Fullwidth” and the grid style is “Masonry”.
The pagination style is “Load more” which is my blog default style.

The first load of the page (which means the first 16 posts) is CORRECT,
which means that those 16 posts are the posts under category “iOS”.

However, after I clicked “Load more posts” button,
the second load of the page is WRONG.
It shows the second load of my front page (which is the 17th-32nd most recent posts),
instead of the 17th-32nd posts under category “iOS”.

My website is http://www.sibutech.com/ and the category page i took as example is http://www.sibutech.com/?page_id=7239

I hope you can look into the problem as soon as possible.