It's kind of hard to say. These are tricky to troubleshoot. If you start the importer over, in theory it should grab the next batch of posts.
In any case, you should go ahead and use Posterous's backup tool to fetch your content, as the service is actually going away in a couple of months. You can use the backup tool to import into your blog using the WordPress (not the Posterous) importer. The backup file currently has some bugs that the WordPress.com importer works around. So in a pinch, you can download the backup file, import into a WordPress.com blog, export from the WordPress.com blog, and import into your self-hosted blog from there.
It's a bit of a hassle, but if you're having trouble with the importer that uses Posterous's API (as it seems you are), it may be worth trying.