Have you waited a while, before checking the database table? Data that is to be logged is kept in memory, before it is written to the database. This process is executed every 300 seconds (which is configurable with the tasks.log.timeout property, which takes a value in milliseconds).
I waited 2 days and the ofMucConversationLog table is always empty, but I can use smack api get the muc history messages, I think the messages always in memory and not written to the database.