I am new to Photoshop and working on an ambitious photo retouching. I have gone through 5 layers correcting and adjusting. Thought I was through, then decided to remove a tree branch. I know how to do it, but I can't get at it. In the top layer or new layer, I can't select it or brush it or clone over it. Just nothing happens. I can clean it up in the background layer with the others turned off, but when I turn them back on, the branch comes back. What is blocking me from selecting this branch?