The navigation for the thumbnails do not always scroll left and right when using the navigation arrows on the desktop. If the nav arrows stop responding, you have to start clicking on thumbnail images and then retry the nav arrows. See link below:
Also, some product galleries do not load all images and at proper thumbnail size. The product below has 1 featured image and 1 gallery image. The featured image thumbnail is loaded much too big and the other gallery image didn’t load at all.