Product suddenly not showing in customer's download area - related to attributes?

I'm at a loss here. I received an email from a customer who purchased several items from my store. She wrote to tell me that two of the items are not available to her for download.

When I checked the customer's order in admin, I found that the order lists all the products, but only one of them is missing from the download area. I have checked the product attributes and everything seems to be ok there. I checked the download manager and it seems ok there as well. The 'missing' product has been purchased by others and has been downloaded in the past, so I'm really stumped here. In fact, the product in question is one of my most popular products.

While I am asking about this... is there any way to add a product to the customer's download area?