No this isn't a joke or troll thread.
Try the following steps
(1) attack or rob a rival and wait 30 seconds
(2) if the boss hasn't been summoned exit the game and try again.
If that doesn't work then try....
(1) do a job
(2) same as above
If that still doesn't work then try
(1) attacking a rival 5 times
(2) same as above
or
(1) Do two jobs
(2) same as above
or
(1) attack a rival 10 times
(2) same as above.
Try force closing your game and / or switching your phone off as well.
Sorry but that's the best advice I can offer if you want to summon more bosses.