If the Scrollmap doesn't cover the whole page (all the way down to the bottom) it might have been run too fast (before all your content has been loaded). To correct this you can re-paint the Scrollmap.
This can be done by opening up the browsers developer tools -> make sure you are in the Element tab and then hit cmd/ctrl + f and search for .heatmap and find the <sg-tag class="heatmap"> element and then select that and delete it.
Then click the console tab and run the following to re-run the Scrollmap function which will draw the Scrollmap once again (now for the entire page).
window.sgheatMap.showScrollmap(window.sg_heat_data);