Just a quick pro tip to anybody making assets (Especially UI) for Unreal and exporting them with Photoshop Generator. Unless you specify a .png quality level, Photoshop will automatically compress the files as much as possible up to and including indexed 8bit color mode.

It turns out that Unreal can't import alpha channels from indexed .png files - so to make everything work you will need to manually mark each layer to export at full quality. To do this, ammend a "32" to the end of the layer asset.

AssetLayer.png > AssetLayer.png32