You may have a pullet that just isn't maturing--it happens, sometimes it is just an undeveloped reproductory system. Certainly not common. My guess is that she maybe laying and that the Minorcas just haven't hit their regular production rate??????

If you examine the vents of both black ones--at night, for easy catch, you can see if they are both laying (vent will be moist, soft and wider than deep). Then look at the vent of another pullet that you know is laying. This is sure way to know if one is not laying.