Problem in range doppler terrain correction

I used from range doppler terrain correction in snap for ALOS2 quad data terrain correction. The result is not reasonable and some part of my image has deleted. I used from all types of DEM and my external DEM. But I faced with this problem again. By the way, my DEM is bigger than image. Do you know what is problem?

These parts are not not deleted, the image has just been adjusted to the local topography. Basically every SAR product acquired over hilly topography looks like this after terrain correction because the pixels are shifted to their correct location (initially displaced by foreshortening or layover effects).