You don't have to use the always build. What happens, anytime you make any changes to the portal settings, the cache is flushed and rebuilt, so it would have updated if you would have just cleared your cache as well.
More than likely it was just timing. If you post and the cache expiration hasn't been reached it won't rebuild. Once the cache expiration has reached it won't rebuild again until another action is triggered to rebuild the cache, in this case someone making a post.
It's a bit misleading in how the cache works. It doesn't auto build every 5 minutes when it's set to 5 minutes. It's all triggered by user interaction. If nobody posts on your site for a half-hour the cache won't be rebuilt for a half-hour.
