Coal?

[kwiedes]

Im trying so hard to get coal and my villagers just seem to not be able to do it. At first when I set up my mine, they gathered coal for about 2minutes. ( I only have 5 workers as of now) and then they stopped.
It had said Production limit reached, which i thought there is no way that could happen, there is no way they mined 200 fuel in 2 minutes.
I set them up to mine iron, I fast foward 2 years, and tried to mine coal again, still production limit reached error. What is wrong?