theoretically, only the white areas in your coherence image reveal usable information. Then there are still atmospheric disturbances left which could affect your fringes. Try to apply a coherence mask on your interferogram as described below to see if the areas of interest are within that areas.
If the areas of interest are excluded by this mask, you need better coherence. You can achieve that by using an image pair with smaller temporal baseline (6/12 days are currently the smallest for Sentinel-1). You can also try to apply a better co-registration in order to improve the image alignment.