It may be hard to tell about depluming mites, versus feather picking, rooster damage, or molting. Molting usually happens at about 16-18 months, then yearly thereafter. Stress can bring about a molt. If you have given a sufficient amount of ivermectin twice 14 days apart, it should have treated...
Did you have the feather mites confirmed by a vet? How much and what strength was the ivermectin? Was it applied on the back of the neck over then spine? Do you have any photos?