Sometimes if a condition is satisfied by multiple objects at once, it will only be triggered once, depending on the type of condition or which object plays which role, ie
On Enemy collides with Bullet
Or
On Bullet Collides with Enemy
I think one of those two will work the way you want it to. Don't know for sure without testing tho.
A surefiire way though, is as a subevent for the condition that kills the enemy do a For Each Object
On Enemy collides with Bullet
----System - For each object : Enemy
-----------------destroy, and increment death count