The excellent How To Make A Platform Game tutorial taught me how to use invisible sprites at the edge of platforms to cause enemy sprites to reverse direction rather than falling off. While trying to implement it in my own game I found that my enemies would respond to the sprite on one edge but ignore the other.
I've attached a simplified example of what I mean. In Example.capx, the yellow sprite should bounce between the two red sprites but it only acknowledges the first one. I would appreciate another pair of eyes looking at the event to point out where I am going wrong.