The triggered event "obj_bullet_yellow On created" is placed as a sub-event under "Every 0.9 seconds". This is wrong. Move it to top-level and it will work.
Also, you code is a bit messy.. You have Enemy family, but you are not utilizing it. You have lots of duplicate events for each enemy type - this is exactly what families are supposed to optimize.
Move LoS, Pathfinding and other behaviors to the family, and you'll be able to decrease the amount of code significantly. See this link:
scirra.com/tutorials/535/how-to-upgrade-an-object-to-a-family
You can add another copy of Enemy family to use it in events like Enemy is overlapping EnemyCopy. You can use "Create by name" action to use in events like Enemy has LoS to player : System create "obj_bullet_"&Enemy.Type and so on..