Hi,
ok, you found a bug, I can repro, basically, the action "collision event" does not trigger within a sub fsm. So for now I would suggest that either you create your fsm as a reaction to the trigger event declared in the parent fsm, or else you use the actual global event ONTRIGGERENTER which works within a sub fsm, I tested. you can then filter triggers if you want.
I filed a bug, so It will likely be fixed on the next update.
bye,
Jean