Suddently Can't Instantly Adjust Exposure of Drawings with Mouse Dragging

I’ve been animating in Harmony for about half a year now. Typically, I’ve gotten used to quickly adjust the exposure of drawings in layers by hovering over them with the mouse in the timeline, causing the edges of frames to highlight in red or yellow. Today, however, hovering over the edge of frames no longer highlights them, and dragging drawings with the mouse no longer changes exposure. I’ve looked for an option to get this back in preferences, but I’ve found nothing that matches the description. Any advice on what to do?