You have a combination of issues going on here. Water rarely works out unless it is dead still and clear enough to be able to see to the bottom. That explains everything on the north side.
Trees are usually an issue at 75% but since some turned out it is likely that the elevation was the cause of the loss of overlap for missing southern images. This is why we offer our Terrain Awareness feature to make it so higher terrain doesn't get less overlap that lower terrain.