Let me start by saying, very neat idea and fun game.
I took a look at it and got it working for you. The problem with the way you were doing it is that you were creating the sprite that made the orb but never destroying it. Yes, you had it set to destroy outside layout, however, it never technically left the layout. It just kept growing and growing and growing....
I used your explosion variable and set it to 0 at start then had your hitpoints >= 6 event turn it to 1 which triggers the orb then waits 2 seconds and then destroys it and sets explosion back to 0. I also reset the explosion variable if you die and reset game, this way its always ready to be changed by bomb trigger event.
I didn't like how the original 4 instances were always coming on screen at start of game from same place so I corralled them for you where they are terminated when game starts (they only have to be present when level is loaded). You can always free them and delete the kill sprite if you don't like it.
Since I played the #$^% out of your level to test my work (and its kinda challenging if you miss the first 5 red orbs) I have the following critique:
1. When I made red orbs worth 2 health to make debugging easier it seemed more fun... maybe make them worth 1.5 will feel same and work with existing art. Try it and see what I mean.
2. With the enemy orbs never destroying themselves it eventually gets too crowded to move, maybe make a % chance when two orbs of the same color collide they destroy each other.. this might allow player to last long enough to set off more than one bomb.