One of the really nice features of Storyline is its support for layers. You might think of them as transparencies that can be overlaid on your base slide. On the transparencies you can place images, sound, and even logic (in the form of Triggers). Each layer has its own independent timeline.
Layers are primarily used to add interaction to your slides. When a button is clicked or the mouse hovers over an object in the base layer, it might trigger the display of a new overlying layer.
If your layer simply shows a static graphic or other image all is well. Every time you display the layer the image will appear. When you hide the layer, the image goes away. In simple presentations, this may be all you need.
Layers have their own independent timelines. This is where the trickiness comes in.
Imagine that your new layer temporarily shows some image or plays some sound. Maybe the layer has its own narration. After the layer’s timeline has completed, there is nothing left on the screen to let you know that it is still active. If you had left some static image on the layer, you would see that. But a transient image or sound comes and goes on its own. You don’t have a clue that the layer is still being shown.
If you later show the same layer again, nothing happens. This can be pretty confusing and difficult to debug. The key is to remember the layer’s timeline. If you show a layer that is already active, nothing happens. The trick is to make sure that this “invisible” layer is closed when execution of the timeline completes. Then, when the layer is shown again, the timeline restarts from scratch and it shows its transient images and plays its audio.
Layers can be explicitly closed by adding a trigger. This trigger’s action would say to “hide this layer when the timeline completes.” Another way would be to click the “gear” icon associated with the layer (in the Layers pane) and check the box (as shown here).