CiviCRM not creating Wordpress user accounts on member registration
+1 for this question - I have had the same issue and I don't have the skills to pin it down. We have a "members only" section of our website and so having up-to-date WP Users is quite important. As a workaround, I go in about once a month and export "current" members from CiviCRM and then import them into WordPress to ensure that there are WP User accounts for each. It's messy but...

I thought it was just a bug with how we've got CiviCRM/WordPress setup (we're amateurs and muddling our way through!). 

(Sorry this isn't a comment - I don't have enough cred yet to post them :). But it's an answer, just not a good one!)