Pro Sites Status Withdrawn Daily

I've got a problem with manually upgraded users having their pro status removed after a day or so of adding it. It's been doing it since we set it up a few weeks ago. So far it's only happened to manually upgraded users and not PayPal ones. I've asked the users to clear their cache etc, but they haven't even been visiting their blogs at the times that it has been withdrawn so I'm not sure what is causing it.

Screenshot attached of recent withdrawal times.

Using WordPress 3.3.1 and Pro Sites at newest version.