Gab Goldenberg has some good advice for how to treat category pages in WordPress. It also applies to ecommerce sites. The post, Category SEO for WordPress Blogs and Ecommerce defines the problem as

By default, Wordpress category pages show 10 posts. Similarly, most ecommerce category pages show 9 products per page (in a 33 grid).

This sucks for SEO, because your individual blog posts or category products get less and less internal linklove over time as they get pushed to the secondary category pages.
Some of the solutions involve:

changing a setting in the All in One SEO plugin if you use it
setting category pages to display up to 100 posts instead of 10 so links to your posts don't get pushed further back in archive
consolidating some posts that deal with the same subject
using one and only one category per post

More details on each in the post. These changes aren't difficult to implement, but they might help bring traffic back to old posts that used to rank well, but no longer do.