If you're finding the order listed in your Transaction Log, that confirms we did process the order for you (issue download links, send emails, log the order details). I also checked your product settings and confirmed you have not disabled the product-specific thank-you email that would be sent for sales of that product. We send emails instantly when we process the order, so the problem apparently comes down to email handling by the receiving mailserver.
Check your spam/junk/bulk mail folder and ask your buyer to do the same, and be sure to mark the message as Not Spam if you find it there. Email providers may also have a delay between accepting an inbound message and actually sorting it for delivery to the recipient's inbox.
Comparing your Transaction Log and Free Downloads Log and taking a guess at which buyer this issue pertains to, it appears she finally received a link from her original purchase and made download attempts as recently as an hour ago.