Categories and Tags are treated the same way by WordPress.com and by search engines. The posts displayed in the global tag pages are displayed in order of their original date stamp. However, the display of posts in the most popular categories/tags pages is randomized as there are 31 million producing 7 posts every minute. Please see here and be sure none of the reasons apply to your blog posts.
missing posts > http://en.support.wordpress.com/global-tags/#missing-posts
Thanks for replying so quickly. But I am still not understanding why they don’t show up.
They don’t show up in the global tags at all. I don’t tag more than 10 tags. I make sure that the tags are all relevant to my post, my blog is not private, and I do not have mature content. My posts used to always show up in the global tags but my last 5 or so have not been showing up.