Thanks guys for answering my previous question. I have some follow up on it. I did mentioned that I have changed my enderman farm design since then and have new problem with it.
So here is the problem, I have a row of extended pistons but they are not powered as you can see from the unpowered redstone behind the pistons.They supposed to be retracted so that the enderman could spawn.
It seems that the pistons are not updated. they only retract when I put a block above the row of pistons.
Is there any ways to solve it without having to climb up onto the spawning spawning and updating pistons every time? Hopefully this question is not too localized.