Tap the layer button at the top-right corner, if you can’t find the layers in the editing interface. Tap on any layer to open a pop-up menu. You will find an “Eye” button in the menu. Tap the button to hide or show the layer.
When working on an animation, you sometimes might want to hide layers to keep the drawing interface clean and simple. However, the hidden layers won’t show up in the exported videos and GIFs.
If you wish to show those layers in the final work, please enable the layer display before exporting the animation.