All reasons for non-appearance of posts on Topics pages can be found here > http://en.support.wordpress.com/topics/#missing-posts
If you have assigned a combined total that exceeds 15 Categories/Tags to your posts, you need to edit and delete. http://en.support.wordpress.com/posts/edit-posts-screen/ After removal of the excess categories and/or tags note it may take several days for your posts to begin displaying there.
You are inadvertently spamdexing AKA tag spamming. And, you ought to be more concerned about your posts not appearing and being well ranked on Google and Bing SERPS (search results pages) than not appearing on the WordPress.com Topics pages because it's search engines that send a significant flow of traffic to blogs and they can choose to bury your content where the sun don't shine for tag spamming.
The rule of thumb is to assign the least, not the most, combined number of only relevant categories and tags that accurately describe the post content.
For tips on tagging see > http://onecoolsitebloggingtips.com/2013/03/15/quick-blog-post-tagging-tips/