It’s hard to tell from what you’ve provided, mainly because we wan’t see where the toggle object is. However, it sounds suspiciously like there’s an enemy that’s in contact with the toggle object when a second one collides with it.
Because the first enemy is still colliding with it, the event won’t trigger and subsequently the second enemy won’t turn. And so there will always be an enemy touching the toggle object, and the event will never get fired again.
Note, the trigger once applies to the object type, not each instance of the object.