This happens because the scroll area is not a browser viewport scroll. To confirm, scroll down the page and look for a full viewport scrollbar on the right. For instance on this page, its easy to see.
Likely there is overflow scroll trickery going on within a content area. The screenshot tool will scroll the viewport scroll, but not this content area overflow scroll. There is no viewport scroll, so the tool thinks there is nothing additional to screenshot.
You can make the screenshot tool work again, by removing the css that makes the overflow scroll on the content div. Use the browser dev tools and just uncheck the css.
There will likely only be a couple things. An overflow hidden on a parent, then scroll and height assignments on children.
Once removed, the page will scroll like a normal page, then the screenshot tool will work.
Its annoying, but once you figure it out its pretty quick.
Full Page screen capture
.