Yes. You can nest BBCodes as long as they are different types. You can have UL BBCode and inside it, you can have LI BBCodes.
And No. You can’t nest the same type of BBCodes. You can’t have UL inside another UL. Ordered or unordered list in HTML is allowed, and you can have UL, inside it several LI tags, and in those LI tags, you can have another UL. This will not work in WordPress, and BBCodes written like that will also fail to render properly. This will not change until WordPress fixes the issue.
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.