Hi,
There's a theory that seems to be working pretty well on several worlds.
Whenever there are more active players close to the respawn of the bosses, the boss tends to respawn more often. As you may know, if you're two floors over the floor where it respawns, the map isn't active and mobs don't move. So the more often the map gets active the more chances of the boss spawning. I would recommend checking the spawning spots every now and then and killing the mobs(just in case this is also relevant)
Seems that some worlds trying this are getting more boses, hope you can help prove this theory. :)