In my previous post I have explained about clearing cache in Gallery.
phpBB Forum is also using Cache for improving performance. So, most of your changes in the coding/database won't reflect immediately.
So, we need to clear the cache whenever we are making any changes in the forum.
For doing this, we should delete all the files available inside the cache folder in the forum.
Or, we can click "Run now" button available near "Purge the cache" under Resynchronise or reset statistics in ACP (Admin Control Panel)