There is a large topic on this is scenarios
here. I know (or think, at least) you are not trying to win the scenario, but get the Gold Rex award, but the topic is basically the same.
You will see this tip there, but I'll tell you now anyway. By the looks of your exhibit, this may be the problem.
Lower your exhibit 3-4 sqaures into the ground and replace your electic fencing with normal. They won't be able to electrocute themselves this way.
If you still can't get it, post over there.
:closed