moving the page bottom also drags elements that are “follow footer” with them, so if one of those managed to go all the way to the top of the page, the page bottom would be unable to move further up.
Another possibility in multi-device contexts is if you have made an element hidden on the current device, it will block the page bottom. You can use the “hidden elements” eye icon in the format bar to show elements that are hidden from the current device.
Finally you might have a completely invisible element, either because it’s transparent or because it’s white like the background. Pressing CMD-A to select all would show the drag handles, revealing it.