The latest episode, number 15, is the only one not showing in the Store - it appears when subscribing: it was only posted yesterday, so it's not at all unusual that it hasn't appeared in the Store yet.
When you add a new episode to your feed and upload it, it appears immediately for subscribers because their iTunes application reads the feed directly and the Store is not involved.
However because of the huge number of podcasts in the Store (thousands) the Store caches the feed and checks it for updates periodically. For this reason it usually takes 1-2 days for new episodes to appear in the Store: sometimes it can be less, and occasionally the whole process seems to get stuck and it can take several days.
You should check your feed by subscribing, either from the Store page or manually from the 'Advanced' menu in iTunes, to see that your new episodes are there. If not, then you have an error of some sort (such as a duplicated 'guid' tag). If all your episodes appear when subscribing then the Store should catch up after a bit.
Right now there are a bunch of minor issues with your feed.
Since you are hosting on Libsyn - you could just use the libsyn on-publish feature to publish to wordpress and then point feedburner to your libsyn feed.
Here is the instructions for setting up Libsyn to publish to your self-hosted wordpress blog.
This would then give you a better RSS feed to point feedburner to.
email me if you have additional questions - rob at libsyn dot com
Just to be clear, those errors won't have any effect on the Store updating. Only one of them actually matters to iTunes, the commas between keywords one on line 143. But no errors is always better, if only for tidyness.
FeedValidator is useful but needs to be approached with caution: it is validating XML feeds, not specifically dealing with suitability for iTunes. It will often throw up errors which only matter with other feed readers, and can miss things which can upset iTunes.