That's because the bloom is dynamic meaning it adjusts automatically depending on brightness of the environment so what I suggest to see it works is:
Make a dark room
Make a REALLY bright room
Put In a door to connect the two
Place the player Start marker in the dark room
Enter the test game
Open the door to the bright room
You should see a big burst of bloom as you first see/go in the bright room and then it slowly adjusts so you can barely notice it.
That is until you go back in the dark room and then back in the bright room and then it will happen again.
Hope this has helped wile not being too confusing.
Mass Effect 2>Tron Legacy>Mirrors Edge>FPSC>Everything else.