Pond with Koi's?

Question

I have a pond that contains 6 Kois. I have had them for 5yrs now.
We bought them when they were about an inch long and now they are about a foot or bigger in length.

How come they haven't mated yet?
How do you tell them apart, male and female?
I know the duller colors are usually the female, what are the chances that 6 out of 6 are all the same sex?
Is there something that we can do to help reproduce?

We have hiding places, plants and such in there, but we never ever see any eggs, or hatchlings.

Answer 1

i hope this help good luck:

http://www.rothteien.com/archives/koi/koibreeding.htm