If you take a look at your photos in the grid view, you'll see that the problem photo (the one with mostly water, and then green grass on the left side) is a lot darker than the photo that it is linking into. I'm not expert, but I would suggest brightening up that photo with some photo editing software. You should check that photo against the others to see that the light levels for all photos are generally the same.
Try messing with the brightness levels and then resynthing your project. I have a feeling it might help.
Don't forget to make a copy of your old photos before you start, though. Sometimes you can overdo it and find that it's synthing worse than before. I've had my far share of that.