The steps for enabling asset downloads for layers depends on whether you're using Sketch or Photoshop with Craft Sync.
Enable asset downloads in Sketch
To enable asset download for layers within a symbol, the layers must be marked for export in Sketch.
To mark a layer within a symbol for export:
Select the layer within the symbol.
At the bottom of the Sketch inspector, click Make Exportable.
Choose the export settings.
Repeat this step for each layer within the symbol you want to generate an asset for in Inspect.
Once all layers within the symbol have been made exportable, 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 the Assets tab. To learn more about Inspect, check out this article: Inspect for Craft Sync
When using Sketch, If you have multiple groups or layers at the same level that have the same name, InVision will automatically append unique numbers to the end of the file names. This allows you to more easily distinguish between similar items available for download. For example, if you have two layers in Sketch named rectangle, they will be renamed rectangle-1 and rectangle-2 in InVision. The layer names in your original Sketch file will remain unchanged.
Enable asset downloads in Photoshop
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 the Assets tab.
For each artboard in Photoshop, we recommend giving a unique name to every group, nested group, and layer that you want to export as assets. If multiple layers or groups share the same name, it can prevent assets from being generated properly in Inspect.