Additionally, a few bosses only respawn after exiting the cave. I don't know why it happens, but if a particular boss does not spawn, even though the other bosses have spawned and respawned extensively (at least over 6 times), then it might be best to leave the cave, your entire group, and enter again. It has worked for me in the past. Re-entering and killing the spawned bosses should allow you to find every boss eventually.