Zbrush alpha problem


Hi, I have sculpt my model, (a man head) on Zbrush. The model have 11 ml polygons. After that I export the mesh on Blender and do the retopo. I import the retopo mesh on Zbrush, subdvide and project all the details into it. Now the model have 8 ml polygons, the problem is that when I try to give details, (I want to add alpha skin pores from texturingxyz) the alpha it’s show pixels. What’s the problem? I mean the retopo isn’t the best but I think it’s not the problem. !

That’s the topology off the mesh