Hello again Rick,
On checking the cache file, the island does build from time to time (I think when a new topic/reply is made on the board) but not at the 5 minute intervals I set it to build to.
Note, that the post islands will only actually update when someone posts to that particular forum. So, even if you have it set to rebuild every 5 minutes, if someone doesn't post in that forum, it won't rebuild until they do. The main reason for that is there's really no reason for a post island to be rebuilt if there isn't a new post in the forum