The scroll bar only scrolls the posts up and down if the posts don't fully fit inside the box. The box doesn't move so if it's too big the bottom is off the screen. Ideally it needs to be repositioned higher up the screen or be a smaller size, (posts will then scroll). You're not the only site to have this issue. My former bank had a similar on-line box that asked for password digits when shopping online. The box removed the 'enter' command when using larger font sizes.