Do low health units not fight?

NoobNoobTrain wrote:

I had this stack of units that all were in red health. I noticed that their combat clock stopped ticking down, so it seems that red health unit do not fight back? Or is it only if the entire stack is red?

The stacks fighting effectiveness will be lowered but will continue to be able to attack until they are wiped out. In your case, it may be this or wiped out just that you display is not refreshed.