There are few reasons for BBCodes not working:
You have disabled BBCodes in the plugin.
You have disabled some of the BBCodes in the plugin.
You are using some 3rd party plugin that is causing problems.
The plugin that is often causing problems with BBCodes is ‘bbPress Advanced Statistics’, and this plugin will disable all Shortcodes/BBCodes by default. Either configure it properly to allow shortcodes, or remove it.