Black line when changing focal point within cropping tool

Hello! I set up image placeholders in Slide Masters to save time on cropping images. However, when I add an image and try to shift the focal point within the crop tool, I'm getting a black line. I believe this is because the image shifted up or down when I moved it left or right and there's some empty space. It's super hard not to shift it though and to find the right spot where no line will be there. Is there a keyboard key that can be held down to lock it vertically while I shift it horizontally? I'm using Storyline 360.