That does seem very unusual. I find 2-3 rumour bosses spawn every 1 hour of real time. It has nothing to do with personality and gossip btw. That just increases the chance of you hearing about it.
The rumour boss will spawn even if you haven't heard about it at an inn, it's possible to accidentally stumble across them in the game world without hearing a rumour.
Once you are lvl 11 and have shards of fate active they should spawn just fine. Like I said, 2-3 should spawn every hour real time.
Thats not in my case. I also dont know what happen. I cant find 3 more shards if there is no rumour bosses. 9 shards i got while in dungeon.