To enable asset download for layers within a symbol, the layers within the symbol must be marked for export within Sketch. To mark a layer within a symbol for export within Sketch, first select the layer and, at the bottom of the Sketch inspector, click Make Exportable.
From here, you can choose the correct export settings and export your layer. Repeat this step for each layer within the symbol you wish to generate an asset for in Inspect.
Once all layers within the symbol have been exported, sync your artboard containing the symbol via Craft Sync. This will generate the layers as individual assets in Inspect mode. You can then download the assets within Inspect mode under Asset Manager. To learn more about Inspect, check out this article: Inspect for Craft Sync
If using Photoshop, the layers within the symbol can be marked for export by adding".png" or any other file format to the end of the layer name. When the source file is synced via Craft Sync, InVision will generate the layers as assets in Inspect mode under Asset Manager.