I would say having just looked at your site it’s an internal linking issue, as you’ve said it looks like Google is seeing your latest post at the more important page.
There are a couple of option I can think off which shouldn’t be to difficult to implement (I’m assuming your using Word Press).
1: create a manual menu and add the links to the tags page you want to rank and try adding that menu to the side bar, this should be relatively simple to setup.
2: (this is my preferred option but it’s a little harder to setup) turn the tag pages your using into actual pages and use a little bit of script to pull through the relevant posts you’ve done, you can then include those pages in your navigation or again in the side bar, this (if done correctly) would mean your loss the “tag” from the URL and because it is then included into the navigation or side bar would be given more weight in Google.
The other thing I would do I link to the URL itself either in each post, something like “see more article about XXX here” at the end of the post or find a nice natural way to link to it in the article itself, I personally don’t really like tag clouds or tagging on post as it can be seen as being a little spammy as you end up with a group of links with target keywords listed at the bottom of each post.
Let me know if how you get on or if you need any more advice.
Thanks
Rufus