By default a simple moderator can not lock topics.
Please verify the Permission masks of a simple moderator not able to unlock topics.
ACP - PERMISSIONS - View global moderation permissions - <username> - Topic actions ==> Is "Can lock topics" set to Yes? If not click the little icon before "Can lock topics" and show us the results.
It would make sense if a simple moderator cannot reopen topics, there is simply no such option. I just needed to make sure there wasn't. If it isn't possible to change that then I'm gonna leave it at that.