Make sure it is really unburned coal you're seeing. Often the ash will contain darker pieces that appear to be unburned or partially burned coal but are actually pieces of rock.
If it is coal you can try increasing the MIN setting. I've seen people have the MIN turned down so low that the fire is almost non existent on the sides of the grate while there is still a sustainable fire through the middle section, letting partially burned coal make it through. In these cases increased combustion air can help, but there is a point where it is too much and will cause the low feed fire to burn through too fast and go out. It is a careful balancing act when trying to eek out optimum idle fires, and you have to be sure the adjustments to combustion air that you make do not cause either an under or over fire condition at the MAX setting.