my subscriptions don't appear to be expiring!

I have set up 2 levels on my site
"stranger" has access to a login page but is blocked from viewing all categories
"subscriber" has access to view all the site

I have 2 subscriptions...
"1 day" which is set to finite, 1 day, like this...
http://twizzlebirdsend.s3.amazonaws.com/q.jpg
and "7 day" which is the same except for set to 7 days.

I set up a user yesterday and manually assigned them to the "1 day" subscription, during this time the user has full access to the site. The subscription has now expired, I can see that if I look at the edit members page expires column confirms this. But the user still has access to view the site. I'm sure I've got something wrong here!

If I manually "drop" a user from the 1 day subscription their site access is revoked, just not when the expiration date is met.

Any help would be massively appreciated.