Are the object(s) in the room. That's often a reason why you only see a black screen,
example.The easiest is probably using a single global variable that you decide how it's set.
Eg you could draw invisible rectangles in the room, and give them a number that correlates to the monster enumerator eg:
monster = e_mon.slime;
Then in the collision event, with the player, you set the global variable to the monster variable
eg
global.monster = monster