Make sure that under your plugin on the gallery settings tab that you have
Download Original Allow users to download original image if image size selected is not available.
Also make sure on your price list under the plugins pricing section that you expand the price list assigned to the gallery and ensure you have the images you want to download set to a “px” for size instead of “in”. Then make sure that you have the check box next to that item in the prices list checked so that it is set to downloadable in the price list.