because you cannot cache the wp page the forum is on... caching is for static content..  the forum is dynamic (changing all the time with new topics/posts)...  so caching it wont allow new stuff to show until the cache is regenerated...  all forum content appears on that single wp page... appearance of many forum pages is just url rewrite 'black magic'...  just dont cache the wp page the forum is on...  all caching plugins we have run across allow you to exclude specific pages... any plugin that generates dynamic content will have same issue...