Coregistration is not perfect

Dear all, I already tried to use Sentinel - 1 Toolbox to make change detection from 2 GRD images. To make it I must coregistration 2 GRD. I usually using auto coregistration to coregister the data. But somehow the data is not perfectly coregistered and there is some blank area in the images (black color in bottom and right images). I suspected that the problem happened because wrong in stacking progress so I tried many resample methodes, but the result is still same. How I supposed to do perfect stacking and coregistration of 2 GRD images? I must do it because my focus areas are in 2 different images.