Paged doesnt work on category pages

Hi!
I need to use “pagenavi” for category pages. What im trying to do is a unique category.php page and show only the posts of each category… the problem become when I write ‘posts_per_page=5’… If i go to http://blabla.bla/category/category_name/page/2 it doesn’t work!!, but if I go to http://blabla.bla/page_name/page/2 it works…. so, the problem is in the categories . What can I do? because i dont want to make one page for each category…

thanks!!