Thanks all for the input so far but perhaps I am not explaining it well.
I am aware of the repair requirement and its set up so that the last post (of two) can be repaired. When the zombies pour in and get caught in the fence; it works for awhile but then stops. The last fence post has this little blue electric graphic around the head and I am unable to repair it (yes I have electrical parts). After the night is over I can repair it and it will be at 95/200 for example. So the fence stops working before the last fence post HP reaches 0.
I will grab some screen shots next time it fails and post them as its likely I am missing something important.