When you say not caught by the spam filter, do you mean the spam comments are ending up in your moderation queue, or in the spam comments section?
If they are in the spam comments section, that's where they should be, and you shouldn't really worry about it, Akismet is doing it's job. You don't even have to empty them or delete them, they'll be gone after 15 days.
If they are in your comment moderation section, then all you can do is, click on 'Spam', and Akismet will learn from it. If you don't have the option ticked in Settings -> Discussion that automatically sends new comments to the moderation queue (or a new author of comments) then I'd suggest you do so, so you can control anything that comes up on your blog.
Spam comes in waves, and actually about 80% of all comments made daily on wordpress.com sites are spam, so there is plenty of it around. For more about spam see here: http://akismet.com/how/