The handle is a slider control. Drag the handle itself, tap either side of it, or focus it and use the arrow keys.
If the page scrolls instead of the handle moving, the two images in the pair are different sizes, so the after image does not line up under the before image and the drag falls through. Give both images the same crop and the same dimensions (the table in Image sizes and focal points says 1200 by 1500, portrait, same framing) and the comparison works.
If only one pair misbehaves, that pair's images are the ones to fix. See Before and after.