Are they saying the download link did not work, or that they never got any link in the first place? If they did not receive a link yet, your PayPal account may still be set to require manually accepting all payments; when you accept a payment manually in your PayPal account, you should see the option to automatically accept all future payments. Once a payment is accepted and completed, then PayPal informs our system of that so we can process the sale and issue a download link to the buyer.
If they are saying they received a download link that did not work, you might test your download by using your E-junkie Seller Admin > "Send free download link" to send yourself a link and test that you can download the file and open the copy of the file you downloaded.
To view a product's uploaded-file stats or replace its uploaded file, you can go into Seller Admin > View/Edit Product > Select your product from the menu and click Edit. There on the first screen of settings for the product, in red text towards the lower-right, you will see the filename, size and upload date of that product's file. You can also use the Overwrite Product File button to upload a replacement file.